xfs: fix xfs_rtalloc_rec units

xfs: fix xfs_rtalloc_rec units

Source kernel commit: a0e5c435babd8bb61612d2e4e9b098cacdd825f7

All the realtime allocation functions deal with space on the rtdev in
units of realtime extents.  However, struct xfs_rtalloc_rec confusingly
uses the word 'block' in the name, even though they're really extents.

Fix the naming problem and fix all the unit handling problems in the two
existing users.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Reviewed-by: Bill O'Donnell <billodo@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>