io_uring: refactor io_arm_poll_handler()

io_uring: refactor io_arm_poll_handler()

gcc 11 goes a weird path and duplicates most of io_arm_poll_handler()
for READ and WRITE cases. Help it and move all pollin vs pollout
specific bits under a single if-else, so there is no temptation for this
kind of unfolding.

before vs after:
   text    data     bss     dec     hex filename
  85362   12650       8   98020   17ee4 ./fs/io_uring.o
  85186   12650       8   97844   17e34 ./fs/io_uring.o

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/1deea0037293a922a0358e2958384b2e42437885.1624739600.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>