p9fs: Move UMA zone initialization to VFS module lifecycle

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.

Reviewed by:	kib
MFC after:	1 week
Differential Revision: https://reviews.freebsd.org/D56492

sys/dev/virtio/p9fs/virtio_p9fs.c

@@ -471,14 +471,12 @@ vt9p_modevent(module_t mod, int type, void *unused)
 	switch (type) {
 	case MOD_LOAD:
 		if (loaded++ == 0) {
-			p9_init_zones();
 			p9_register_trans(&vt9p_trans);
 		}
 		break;
 	case MOD_UNLOAD:
 		if (--loaded == 0) {
 			p9_unregister_trans(&vt9p_trans);
-			p9_destroy_zones();
 		}
 		break;
 	case MOD_SHUTDOWN:
@@ -487,6 +485,7 @@ vt9p_modevent(module_t mod, int type, void *unused)
 		error = EOPNOTSUPP;
 		break;
 	}
+
 	return (error);
 }
 

sys/fs/p9fs/p9fs_vfsops.c

@@ -119,6 +119,8 @@ p9fs_init(struct vfsconf *vfsp)
 	p9fs_io_buffer_zone = uma_zcreate("p9fs io_buffer zone",
 	    P9FS_MTU, NULL, NULL, NULL, NULL, UMA_ALIGN_PTR, 0);
 
+	p9_init_zones();
+
 	return (0);
 }
 
@@ -127,6 +129,8 @@ static int
 p9fs_uninit(struct vfsconf *vfsp)
 {
 
+	p9_destroy_zones();
+
 	uma_zdestroy(p9fs_node_zone);
 	uma_zdestroy(p9fs_io_buffer_zone);
 	uma_zdestroy(p9fs_getattr_zone);