fs/9p: check schedule_timeout_interruptible return value

In v9fs_file_do_lock() we need to check return value of
schedule_timeout_interruptible() and exit the loop when it
returns nonzero, otherwise the loop is not really interruptible
and after the signal, the loop is no longer throttled by
P9_LOCK_TIMEOUT.

Signed-off-by: Jim Garlick <garlick.jim@gmail.com>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>