LoongArch: Add support to clone a time namespace

We can see that "Time namespaces are not supported" on LoongArch:

(1) clone3 test
# cd tools/testing/selftests/clone3 && make && ./clone3
...
# Time namespaces are not supported
ok 18 # SKIP Skipping clone3() with CLONE_NEWTIME
# Totals: pass:17 fail:0 xfail:0 xpass:0 skip:1 error:0

(2) timens test
# cd tools/testing/selftests/timens && make && ./timens
...
1..0 # SKIP Time namespaces are not supported

On LoongArch the current kernel does not support CONFIG_TIME_NS which
depends on GENERIC_VDSO_TIME_NS, select GENERIC_VDSO_TIME_NS to enable
CONFIG_TIME_NS to build kernel/time/namespace.c.

Additionally, it needs to define some arch-dependent functions for the
timens, such as __arch_get_timens_vdso_data(), arch_get_vdso_data() and
vdso_join_timens().

At the same time, modify the layout of vvar to use one page size for
generic vdso data, expand another page size for timens vdso data and
assign LOONGARCH_VDSO_DATA_SIZE (maybe exceeds a page size if expand in
the future) for loongarch vdso data, at last add the callback function
vvar_fault() and modify stack_top().

With this patch under CONFIG_TIME_NS:

(1) clone3 test
# cd tools/testing/selftests/clone3 && make && ./clone3
...
ok 18 [739] Result (0) matches expectation (0)
# Totals: pass:18 fail:0 xfail:0 xpass:0 skip:0 error:0

(2) timens test
# cd tools/testing/selftests/timens && make && ./timens
...
# Totals: pass:10 fail:0 xfail:0 xpass:0 skip:0 error:0

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

LoongArch: Fix build warnings in VDSO

Fix build warnings in VDSO as below:

arch/loongarch/vdso/vgettimeofday.c:9:5: warning: no previous prototype for '__vdso_clock_gettime' [-Wmissing-prototypes]
9 | int __vdso_clock_gettime(clockid_t clock,
| ^~~~~~~~~~~~~~~~~~~~
arch/loongarch/vdso/vgettimeofday.c:15:5: warning: no previous prototype for '__vdso_gettimeofday' [-Wmissing-prototypes]
15 | int __vdso_gettimeofday(struct __kernel_old_timeval *tv,
| ^~~~~~~~~~~~~~~~~~~
arch/loongarch/vdso/vgettimeofday.c:21:5: warning: no previous prototype for '__vdso_clock_getres' [-Wmissing-prototypes]
21 | int __vdso_clock_getres(clockid_t clock_id,
| ^~~~~~~~~~~~~~~~~~~
arch/loongarch/vdso/vgetcpu.c:27:5: warning: no previous prototype for '__vdso_getcpu' [-Wmissing-prototypes]
27 | int __vdso_getcpu(unsigned int *cpu, unsigned int *node, struct getcpu_cache *unused)

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

LoongArch: Add vDSO syscall __vdso_getcpu()

We test 20 million times of getcpu(), the real syscall version take 25
seconds, while the vsyscall version take only 2.4 seconds.

Signed-off-by: Rui Wang <wangrui@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>