fs: dodge an atomic in putname if ref == 1

fs: dodge an atomic in putname if ref == 1

While the structure is refcounted, the only consumer incrementing it is
audit and even then the atomic operation is only needed when it
interacts with io_uring.

If putname spots a count of 1, there is no legitimate way for anyone to
bump it.

If audit is disabled, the count is guaranteed to be 1, which
consistently elides the atomic for all path lookups. If audit is
enabled, it still manages to elide the last decrement.

Note the patch does not do anything to prevent audit from suffering
atomics. See [1] and [2] for a different approach.

Benchmarked on Sapphire Rapids issuing access() (ops/s):
before: 5106246
after:  5269678 (+3%)

Link 1:	https://lore.kernel.org/linux-fsdevel/20250307161155.760949-1-mjguzik@gmail.com/
Link 2: https://lore.kernel.org/linux-fsdevel/20250307164216.GI2023217@ZenIV/

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://lore.kernel.org/r/20250311181804.1165758-1-mjguzik@gmail.com
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>