net/9p: return error when p9_client_stat fails

p9_client_stat function doesn't return correct value if it fails.
p9_client_stat should return ERR_PTR of the error value when it fails.
Instead, it always returns a value to the allocated p9_wstat struct even
when it is not populated correctly.

This patch makes p9_client_stat to handle failure correctly.

Signed-off-by: Latchesar Ionkov <lucho@ionkov.net>
Reviewed-by: Eric Van Hensbergen <ericvh@gmail.com>