xfs: perag may be null in xfs_imap()

Dan Carpenter's static checker reported:

The patch 7b13c515: "xfs: use perag for ialloc btree cursors"
from Jun 2, 2021, leads to the following Smatch complaint:

    fs/xfs/libxfs/xfs_ialloc.c:2403 xfs_imap()
    error: we previously assumed 'pag' could be null (see line 2294)

And it's right. Fix it.

Fixes: 7b13c515

 ("xfs: use perag for ialloc btree cursors")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>