sys/kern/vfs_mount.c: Provide mountlist_find_resource() func

Search the vfs list for a specified resource. Returns a pointer to it
or NULL if no suitable entry is found. The caller of this routine
is responsible for releasing the returned vfs pointer.

Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
sys/cddl: Use mountlist_find_resource() in zfs_ioctl.c

Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>