fsmap: fix documentation of FMR_OF_LAST

The FMR_OF_LAST flag is set on the last fsmap record being returned for
the dataset requested, contrary to what the header file says.  Fix the
docs to reflect the behavior of all fsmap implementations.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>