xfs: convert xfs_imap() to take a perag

Callers have referenced perags but they don't pass it into
xfs_imap() so it takes it's own reference. Fix that so we can change
inode allocation over to using active references.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>