memif: handle unexpected EPOLLOUT for RX queue

memif: handle unexpected EPOLLOUT for RX queue

Getting an unexpected EPOLLOUT for RX queue and crashing due to missing
callback function on write_function to handle EPOLLOUT.

(gdb) f 5
f 5
    node=<optimized out>, vm=0x7f948bbe4540 <vlib_global_main>) at /vpp/src/vlib/unix/input.c:325
325		      errors[n_errors] = f->write_function (f);
(gdb) p *e
p *e
$21 = {events = 4, data = {ptr = 0x23, fd = 35, u32 = 35, u64 = 35}}
(gdb) p file_main.file_pool[e->data.fd]
p file_main.file_pool[e->data.fd]
$22 = {file_descriptor = 37, flags = 1, polling_thread_index = 0, private_data = 65536,
  read_function = 0x7f944a87b140 <memif_int_fd_read_ready>, write_function = 0x0,
  error_function = 0x0, description = 0x7f944be22400 "memif0/1 rx 0 int", read_events = 0,
  write_events = 1, error_events = 0}
(gdb)

File descriptors are recycled. It is likely that the EPOLLOUT is meant
for the previous owner of the same file descriptor number which memif
recycled and memif RX did not expect to receive an EPOLLOUT. For
defensive play, we create a dummy callback function to ignore the
EPOLLOUT instead of crashing.

Type: fix

Signed-off-by: Steven Luong <sluong@cisco.com>
Change-Id: I535cadbb9b71d3db2995b118c59a12c71f10af09