selftests: mount: add nosymfollow tests

Add tests for the new 'nosymfollow' mount option. We test to make sure
that symlink traversal fails with ELOOP when 'nosymfollow' is set, but
that readlink(2) and realpath(3) still work as expected. We also verify
that statfs(2) correctly returns ST_NOSYMFOLLOW when we are mounted with
the 'nosymfollow' option.

Signed-off-by: Ross Zwisler <zwisler@google.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>