io_uring/rsrc: skip atomic refcount for uncloned buffers

io_uring/rsrc: skip atomic refcount for uncloned buffers

io_buffer_unmap() performs an atomic decrement of the io_mapped_ubuf's
reference count in case it has been cloned into another io_ring_ctx's
registered buffer table. This is an expensive operation and unnecessary
in the common case that the io_mapped_ubuf is only registered once.
Load the reference count first and check whether it's 1. In that case,
skip the atomic decrement and immediately free the io_mapped_ubuf.

Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
Link: https://lore.kernel.org/r/20250619143435.3474028-1-csander@purestorage.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>