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The pipelined interrupt entry code must always run the common work
loop before returning to user mode on the in-band stage, including
after the preempted task was demoted from oob to in-band context as a
result of handling the incoming IRQ.

Failing to do so may cause in-band work to be left pending in this
particular case, like _TIF_RETUSER and other _TIF_WORK conditions.

This bug caused the smokey 'gdb' test to fail on x86:
https://xenomai.org/pipermail/xenomai/2021-March/044522.html

Signed-off-by: Philippe Gerum <rpm@xenomai.org>