f2fs: use generic checking and prep function for FS_IOC_SETFLAGS

Make the f2fs implementation of FS_IOC_SETFLAGS use the new VFS helper
function vfs_ioc_setflags_prepare().

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>