9p: Possible regression in p9_client_stat

Fix a possible regression with p9_client_stat where it can try to kfree 
an ERR_PTR after an erroneous p9pdu_readf. Also remove an unnecessary data 
buffer increment in p9_client_read.

Signed-off-by: Abhishek Kulkarni <adkulkar@umail.iu.edu>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>