f2fs: remove redundant check from f2fs_setflags_common()

Now that f2fs_ioc_setflags() and f2fs_ioc_fssetxattr() call the VFS
helper functions which check for permission to change the immutable and
append-only flags, it's no longer needed to do this check in
f2fs_setflags_common() too.  So remove it.

This is based on a patch from Darrick Wong, but reworked to apply after
commit 360985573b55 ("f2fs: separate f2fs i_flags from fs_flags and ext4
i_flags").

Originally-from: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>