9p: fix return value in case in v9fs_fid_xattr_set()

In case of error in the p9_client_write, the function v9fs_fid_xattr_set
should return its negative value, what was never being done.

In case of success it only retuned 0. Now it returns the 'offset'
variable (write_count total).

Signed-off-by: Geyslan G. Bem <geyslan@gmail.com>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>