Previously, the UMA zones required for 9P requests (p9fs_buf_zone,
p9fs_req_zone, etc.) were initialized and destroyed in the
virtio_p9fs transport module. This caused issues when unloading
the core p9fs module.
This change moves p9_init_zones() and p9_destroy_zones() into
p9fs_init() and p9fs_uninit() inside p9fs_vfsops.c so that they
are correctly bound to the VFS filesystem module lifecycle via
vfs_modevent, aligning p9fs with standard FreeBSD VFS semantics.
Found while fixing https://github.com/CTSRD-CHERI/cheribsd/issues/2617.