scsi: fc: start decoupling fc_block_scsi_eh from scsi_cmnd

Scsi_cmnd is an unsuitable argument for eh_device_reset_handler(),
eh_target_reset_handler(), and eh_host_reset_handler() which do not have
the scope of one single SCSI command.  These callbacks tend to use
fc_block_scsi_eh() requiring scsi_cmnd.  In order to start decoupling
above eh callbacks from scsi_cmnd, introduce a new variant of the
function called fc_block_rport() taking an fc_rport as argument.
Refactor the old fc_block_scsi_eh() to simply delegate to
fc_block_rport().

Signed-off-by: Steffen Maier <maier@linux.vnet.ibm.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>