f2fs: avoid duplicated permission check for "trusted." xattrs

Because xattr_permission already checks CAP_SYS_ADMIN
capability, we don't need to check it.

Signed-off-by: Hyunchul Lee <cheol.lee@lge.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>