make prepend_name() work correctly when called with negative *buflen

make prepend_name() work correctly when called with negative *buflen

commit e825196d48d2b89a6ec3a8eff280098d2a78207e upstream.

In all callchains leading to prepend_name(), the value left in *buflen
is eventually discarded unused if prepend_name() has returned a negative.
So we are free to do what prepend() does, and subtract from *buflen
*before* checking for underflow (which turns into checking the sign
of subtraction result, of course).

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>