9p: Check for NULL fid in v9fs_dir_release()

NULL fid should be handled in cases where we endup calling v9fs_dir_release()
before even we instantiate the fid in filp.

Signed-off-by: Venkateswararao Jujjuri <jvrao@linux.vnet.ibm.com>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>