selftests: fchmodat2: remove duplicate unneeded defines

These duplicate defines should automatically be picked up from kernel
headers. Use KHDR_INCLUDES to add kernel header files.

Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Message-Id: <20230805073809.1753462-4-usama.anjum@collabora.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

selftests: Add fchmodat2 selftest

The test marks as skipped if a syscall with the AT_SYMLINK_NOFOLLOW flag
fails. This is because not all filesystems support changing the mode
bits of symlinks properly. These filesystems return an error but change
the mode bits:

newfstatat(4, "regfile", {st_mode=S_IFREG|0640, st_size=0, ...}, AT_SYMLINK_NOFOLLOW) = 0
newfstatat(4, "symlink", {st_mode=S_IFLNK|0777, st_size=7, ...}, AT_SYMLINK_NOFOLLOW) = 0
syscall_0x1c3(0x4, 0x55fa1f244396, 0x180, 0x100, 0x55fa1f24438e, 0x34) = -1 EOPNOTSUPP (Operation not supported)
newfstatat(4, "regfile", {st_mode=S_IFREG|0640, st_size=0, ...}, AT_SYMLINK_NOFOLLOW) = 0

This happens with btrfs and xfs:

$ tools/testing/selftests/fchmodat2/fchmodat2_test
TAP version 13
1..1
ok 1 # SKIP fchmodat2(symlink)
# Totals: pass:0 fail:0 xfail:0 xpass:0 skip:1 error:0

$ stat /tmp/ksft-fchmodat2.*/symlink
File: /tmp/ksft-fchmodat2.3NCqlE/symlink -> regfile
Size: 7 Blocks: 0 IO Block: 4096 symbolic link
Device: 7,0 Inode: 133 Links: 1
Access: (0600/lrw-------) Uid: ( 0/ root) Gid: ( 0/ root)

Signed-off-by: Alexey Gladkov <legion@kernel.org>
Message-Id: <4532a04a870ff589ba62ceeacf76f0bd81b9ba01.1689092120.git.legion@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>