net/9p/protocol.c: Fix a memory leak

When p9pdu_readf() is called with "s" attribute, it allocates a pointer that
will store a string. In p9dirent_read(), this pointer is not being released,
leading to out of memory errors.
This patch releases this pointer after string is copyed to dirent->d_name.

Signed-off-by: Pedro Scarapicchia Junior <pedro.scarapiccha@br.flextronics.com>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>