xfs: fix exchmaps reservation limit check

xfs_exchmaps_estimate_overhead() adds the bmbt and rmapbt
overhead to a local resblks variable, but the final UINT_MAX
check still tests req->resblks.  That is the reservation value
from before the overhead was added.

The computed value is stored back in req->resblks and later passed
to xfs_trans_alloc(), whose block reservation argument is unsigned
int.  Check the computed reservation so the existing limit applies
to the value that will be used.

Fixes: 966ceafc7a ("xfs: create deferred log items for file mapping exchanges")
Cc: stable@vger.kernel.org # v6.10
Signed-off-by: Yingjie Gao <gaoyingjie@uniontech.com>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>

fs/xfs/libxfs/xfs_exchmaps.c

@@ -711,7 +711,7 @@ xfs_exchmaps_estimate_overhead(
 		return -ENOSPC;
 
 	/* Can't actually reserve more than UINT_MAX blocks. */
-	if (req->resblks > UINT_MAX)
+	if (resblks > UINT_MAX)
 		return -ENOSPC;
 
 	req->resblks = resblks;