Changes

osmo_io: Fix msgb memleak if iofd is unregistered during write_cb with >1 io buffers

The msgbs are not allocated under the msghdr, hence if user unregistered
the iofd we need to manually free all remaining msgbs when freeing the
msghdr.

Change-Id: I579bc2142bba02947021c47d94bf2fe4f2040b01