btrfs: print checksum type and implementation at mount time

Per user request, print the checksum type and implementation at mount
time among the messages. The checksum is user configurable and the
actual crypto implementation is useful to see for performance reasons.
The same information is also available after mount in
/sys/fs/FSID/checksum file.

Example:

  [25.323662] BTRFS info (device vdb): using sha256 (sha256-generic) checksum algorithm

Link: https://github.com/kdave/btrfs-progs/issues/483


Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>