scsi: ufs: ufshpb: Verify that 'num_inflight_map_req' is non-negative

'num_inflight_map_req' should not be negative.  It is incremented and
decremented without any protection, allowing it theoretically to be
negative, should some weird unbalanced count occur.

Verify that the those calls are properly serialized.

Link: https://lore.kernel.org/r/20210808090024.21721-4-avri.altman@wdc.com
Fixes: 33845a2d

 (scsi: ufs: ufshpb: Limit the number of in-flight map requests)
Signed-off-by: Avri Altman <avri.altman@wdc.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>