[SCSI] zfcp: Simplify SBAL allocation to fix sparse warnings

When waiting for a request claim the SBAL before waiting. This way,
locking before each check of the free counter is not required and
sparse does not emit warnings for the complicated locking scheme.

Signed-off-by: Christof Schmitt <christof.schmitt@de.ibm.com>
Acked-by: Felix Beck <felix@linux.vnet.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>