io_uring: fix deadlock on iowq file slot alloc

io_fixed_fd_install() can grab uring_lock in the slot allocation path
when called from io-wq, and then call into io_install_fixed_file(),
which will lock it again. Pull all locking out of
io_install_fixed_file() into io_fixed_fd_install().

Fixes: 1339f24b

 ("io_uring: allow allocated fixed files for openat/openat2")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/64116172a9d0b85b85300346bb280f3657aafc26.1654087283.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>