v9fs_walk: As per 9p2000 RFC, MAXWELEM >= nwnames >= 0.

v9fs_walk: As per 9p2000 RFC, MAXWELEM >= nwnames >= 0.

The nwnames field in TWALK message is assumed to be >=0 and <= MAXWELEM
which is defined as macro P9_MAXWELEM (16) in virtio-9p.h as per 9p2000
RFC. Appropriate changes are required in V9fsWalkState and v9fs_walk.

Signed-off-by: Harsh Prateek Bora <harsh@linux.vnet.ibm.com>
Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Venkateswararao Jujjuri <jvrao@linux.vnet.ibm.com>