Properly fix out of memory handling in the iSCSI target.

Approved by:	re (glebius)
Sponsored by:	FreeBSD Foundation
In usr.sbin/bhyve/pci_ahci.c, fix several gcc warnings of the form
"assignment makes pointer from integer without a cast", by changing the
cmd_lst and rbis members of struct ahci_port from integers to pointers.

Also surround a pow-of-2 test expression with parentheses to clarify it,
and avoid another gcc warning.

Approved by:	re (glebius)
Reviewed by:	grehan, mav
In sys/amd64/amd64/pmap.c, fix several gcc warnings about uninitialized
variables in reclaim_pv_chunk().

Approved by:	re (marius)
Reviewed by:	neel, kib
X-MFC-With:	r256072