9p: untangle ->poll() mess

First of all, NULL ->poll() means "always POLLIN, always POLLOUT", not an error.
Furthermore, mixing -EREMOTEIO with POLL... masks and expecting it to do anything
good is insane - both are arch-dependent, to start with.  Pass a pointer to
store the error value separately and make it return POLLERR in such case.

And ->poll() calling conventions do *not* include "return -Esomething".  Never
had.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>