preserve 'when' flags of new osmo_fd in ipaccess_rcvmsg()
preserve 'when' flags of new osmo_fd in ipaccess_rcvmsg()

ipaccess_rcvmsg() disposes of a temporary osmo_fd structure after
the RSL link comes up. It copies data from its temporary osmo_fd
to the new one returned by sign_link_up(). However, in doing so,
it clobbered the 'when' flags, which could differ between the two
osmo_fd structures.
For instance, BSC_FD_WRITE could be set on the new osmo_fd but
not on the old one, in case sign_link_up() has already enqueued
outbound messages using the new osmo_fd.

Because of this behaviour, a patch committed to osmo-bsc to address
issue #2719 did not work as intended and had to be reverted.
After this change, that osmo-bsc patch should work as intended
and issue #2719 can hopefully be resolved.

Change-Id: I52f7c903212b38e9c87e4d45e52b231b6f1ae9f5
Related: OS#2719

osmocom/libosmo-abis,osmocom/libosmo-abis,osmocom/libosmo-abis