[SCSI] zfcp: module parameter dbflevel for early debugging

So far, we could only increase the s390dbf log level after an FCP
device has been initially set online for it to create the dbf entries
required to adjust the level.

Introduce zfcp.dbflevel as counterpart to the already existing
zfcp.dbfsize to enable debugging of e.g. setting an FCP device online.

Signed-off-by: Steffen Maier <maier@linux.vnet.ibm.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>