Loosen fchmodat AT_SYMLINK_NOFOLLOW test on symlink.

Loosen fchmodat AT_SYMLINK_NOFOLLOW test on symlink.

It has been reported in b2/19657449 and b2/19381040 that fchmodat
AT_SYMLINK_NOFOLLOW operation on symlink can succeed. It seems to be
controlled by kernel(version or configuration) or user configuration
whether chmod is allowed on symlinks. Unless we can disable chmod on
symlinks in bionic explicitly, we can not guarantee that the test can
pass. But it seems reasonable to allow chmod on symlink if kernel allows
to. So We prefer to loosen the test here, accepting both success and
failure when doing chmod operation on symlinks.

Bug: 19657449
Bug: 19381040
Change-Id: I780e84f0b50d0412fbac9f1c240d07e984892a28