xfs: fix off-by-one error in xfs_rtalloc_query_range

In commit 8ad560d2 ("xfs: strengthen rtalloc query range checks")
we strengthened the input parameter checks in the rtbitmap range query
function, but introduced an off-by-one error in the process.  The call
to xfs_rtfind_forw deals with the high key being rextents, but we clamp
the high key to rextents - 1.  This causes the returned results to stop
one block short of the end of the rtdev, which is incorrect.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>