9p: fix debug compilation error

s/9p/v9fs.c: In function 'v9fs_parse_options':
fs/9p/v9fs.c:134: error: 'p9_debug_level' undeclared (first use in this function)

Signed-off-by: Dave Jones <davej@redhat.com>
Acked-by: Eric Van Hensbergen <ericvh@gmail.com>