9p: fix sparse warnings in new xattr code

fixes:

  CHECK   fs/9p/xattr.c
	fs/9p/xattr.c:73:6: warning: Using plain integer as NULL pointer
	fs/9p/xattr.c:135:6: warning: Using plain integer as NULL pointer

Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>