f2fs: support iflag change given the mask

In f2fs_fileattr_set(),

	if (!fa->flags_valid)
		mask &= FS_COMMON_FL;

In this case, we can set supported flags by mask only instead of BUG_ON.

/* Flags shared betwen flags/xflags */
	(FS_SYNC_FL | FS_IMMUTABLE_FL | FS_APPEND_FL | \
	 FS_NODUMP_FL |	FS_NOATIME_FL | FS_DAX_FL | \
	 FS_PROJINHERIT_FL)

Fixes: 9b1bb01c ("f2fs: convert to fileattr")
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>