fs/9p: fix error reporting in v9fs_dir_release

Checking the p9_fid_put value allows us to pass back errors
involved if we end up clunking the fid as part of dir_release.

This can help with more graceful response to errors in writeback
among other things.

Signed-off-by: Eric Van Hensbergen <ericvh@kernel.org>
Reviewed-by: Dominique Martinet <asmadeus@codewreck.org>