fs: introduce a wrapper uuid_to_fsid()

Some filesystem's use a digest of their uuid for f_fsid.
Create a simple wrapper for this open coded folding.

Filesystems that have a non null uuid but use the block device
number for f_fsid may also consider using this helper.

[JK: Added missing asm/byteorder.h include]
Link: https://lore.kernel.org/r/20210322173944.449469-2-amir73il@gmail.com


Acked-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>