scsi: zfcp: early returns for traces disabled via level

This patch adds early checks to avoid burning CPU cycles on
the assembly of trace entries which would be skipped anyway.

Introduce a static const variable to keep the trace level to check with
debug_level_enabled() in sync with the actual trace emit with
debug_event(). In order not to refactor the SAN tracing too much,
simply use a define instead.

This change is only for the non / semi hot paths,
while the actual (I/O) hot path was already improved earlier:
zfcp_dbf_scsi() is already guarded by its only caller _zfcp_dbf_scsi()
since commit dcd20e23 ("[SCSI] zfcp: Only collect SCSI debug data for
matching trace levels").
zfcp_dbf_hba_fsf_res() is already guarded by its only caller
zfcp_dbf_hba_fsf_response() since commit 2e261af8

 ("[SCSI] zfcp: Only
collect FSF/HBA debug data for matching trace levels").

Signed-off-by: Martin Peschke <mpeschke@linux.vnet.ibm.com>
[maier@linux.vnet.ibm.com: rebase, reword, default level 3 branch prediction]
Signed-off-by: Steffen Maier <maier@linux.vnet.ibm.com>
Reviewed-by: Benjamin Block <bblock@linux.vnet.ibm.com>
Signed-off-by: Benjamin Block <bblock@linux.vnet.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>