IB/ipath: Don't corrupt pending mmap list when unmapped objects are freed

Fix the pending mmap code so it doesn't corrupt the list of pending
mmaps and crash the machine when pending mmaps are destroyed without
first being mapped.  Also, remove an unused variable, and use standard
kernel lists instead of our own homebrewed linked list implementation
to keep the pending mmap list.

Signed-off-by: Robert Walsh <robert.walsh@qlogic.com>
Signed-off-by: Ralph Campbell <ralph.campbell@qlogic.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>