[SCSI] fcoe: add mutex to protect create and destroy

Rather than rely on the hostlist_lock to be held while creating exchange
managers, serialize fcoe instance creation and destruction with a mutex.
This will allow the hostlist addition to be moved out of fcoe_if_create(),
which will simplify NPIV support.

Signed-off-by: Chris Leech <christopher.leech@intel.com>
Signed-off-by: Robert Love <robert.w.love@intel.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>