io_uring: Fold allocation into alloc_cache helper

The allocation paths that use alloc_cache duplicate the same code
pattern, sometimes in a quite convoluted way.  Fold the allocation into
the cache code itself, making it just an allocator function, and keeping
the cache policy invisible to callers.  Another justification for doing
this, beyond code simplicity, is that it makes it trivial to test the
impact of disabling the cache and using slab directly, which I've used
for slab improvement experiments.

One relevant detail is that we provide a callback to optionally
initialize memory only when we actually reach slab.  This allows us to
avoid blindly executing the allocation with GFP_ZERO and only clean
fields when they matter.

Signed-off-by: Gabriel Krisman Bertazi <krisman@suse.de>
Link: https://lore.kernel.org/r/20241216204615.759089-2-krisman@suse.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>