io_uring: fix double unlock for pbuf select

io_buffer_select(), which is the only caller of io_ring_buffer_select(),
fully handles locking, mutex unlock in io_ring_buffer_select() will lead
to double unlock.

Fixes: c7fb1942 ("io_uring: add support for ring mapped supplied buffers")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>