As it turns out, when MOD_LOAD handler returns error, kernel calls MOD_UNLOAD
handler.  Make the new iSCSI initiator not panic when this happens.

Approved by:	re (glebius)
Sponsored by:	FreeBSD Foundation
Don't parse NO_ROOT metadata for extra kernels if NO_ROOT isn't defined.

Approved by:	re (gjb)
MFC after:	1 week