scsi: sd_zbc: Use kvzalloc() to allocate REPORT ZONES buffer

We have two reports of failed memory allocation in btrfs' code which is
calling into report zones.

Both of these reports have the following signature coming from
__vmalloc_area_node():

 kworker/u17:5: vmalloc error: size 0, failed to allocate pages, mode:0x10dc2(GFP_KERNEL|__GFP_HIGHMEM|__GFP_NORETRY|__GFP_ZERO), nodemask=(null),cpuset=/,mems_allowed=0

Further debugging showed these where allocations of one sector (512
bytes) and at least one of the reporter's systems where low on memory,
so going through the overhead of allocating a vm area failed.

Switching the allocation from __vmalloc() to kvzalloc() avoids the
overhead of vmalloc() on small allocations and succeeds.

Note: the buffer is already freed using kvfree() so there's no need to
adjust the free path.

Cc: Qu Wenru <[email protected]>
Cc: Naohiro Aota <[email protected]>
Link: kdave/btrfs-progs#779
Link: kdave/btrfs-progs#915
Fixes: 23a5086 ("scsi: sd_zbc: Cleanup sd_zbc_alloc_report_buffer()")
Signed-off-by: Johannes Thumshirn <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Reviewed-by: Damien Le Moal <[email protected]>
Signed-off-by: Martin K. Petersen <[email protected]>

drivers/scsi/sd_zbc.c

@@ -188,8 +188,7 @@ static void *sd_zbc_alloc_report_buffer(struct scsi_disk *sdkp,
 	bufsize = min_t(size_t, bufsize, queue_max_segments(q) << PAGE_SHIFT);
 
 	while (bufsize >= SECTOR_SIZE) {
-		buf = __vmalloc(bufsize,
-				GFP_KERNEL | __GFP_ZERO | __GFP_NORETRY);
+		buf = kvzalloc(bufsize, GFP_KERNEL | __GFP_NORETRY);
 		if (buf) {
 			*buflen = bufsize;
 			return buf;