net/9p: Use proper data types

Use proper data types for storing the count of the binary blob and
length of a string. Without this patch length calculation of string will
always result in -1 because of comparision between signed and unsigned
integer.

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