aio: Improve UMA usage

• Remove the AIO proc zone. This zone gets one allocation per AIO daemon process, which isn't enough to warrant a dedicated zone. Plus, unlike other AIO structures, aiops are small (32 bytes with LP64), so UMA doesn't provide better space efficiency than malloc(9). Change one of the malloc types in vfs_aio.c to make it more general.

• Don't set the NOFREE flag on the other AIO zones. This flag means that memory allocated to the AIO subsystem is never freed back to the VM, so it's always preferable to avoid using it when possible. NOFREE was set without explanation when AIO was converted to use UMA 20 years ago, but it does not appear to be required; all of the structures allocated from UMA (per-process kaioinfo, kaiocb, and aioliojob) keep track of references and get freed only when none exist. Plus, these structures will contain dangling pointer after they're freed (e.g., the "cred", "fd_file" and "uiop" fields of struct kaiocb), so use-after-frees are dangerous even when the structures themselves are type-stable.

Reviewed by: asomers
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35493