[SCSI] zfcp: Merge trace code for fsf requests in one function

The latencies traced per fsf request are traced for sysfs output and
for blktrace, each in one function. Simplify the tracing code by
merging both tracing functions into one.

Reviewed-by: Swen Schillig <swen@vnet.ibm.com>
Signed-off-by: Christof Schmitt <christof.schmitt@de.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>