[SCSI] st: Increase success probability in driver buffer allocation

Modify allocation to try the minimum possible page order allowed by the HBA
scatter/gather segment limit in allocation of the driver's internal
buffer. This increases the probability of successful allocation. The
allocation may still fail if this minimum order is > 0.

Signed-off-by: FUJITA Tomonori <[e-mail]>
Signed-off-by: Kai Makisara <[e-mail]>
Reported-by: Lukas Kolbe <[e-mail]>
Signed-off-by: James Bottomley <[e-mail]>

drivers/scsi/st.c

···
 		b_size = PAGE_SIZE << order;
 	} else {
 		for (b_size = PAGE_SIZE, order = 0;
-		     order < ST_MAX_ORDER && b_size < new_size;
+		     order < ST_MAX_ORDER &&
+			     max_segs * (PAGE_SIZE << order) < new_size;
 		     order++, b_size *= 2)
 			;  /* empty */
 		STbuffer->reserved_page_order = order;