[PATCH] fs: 9p/conv.c error path fix

When buf_check_overflow() returns != 0 we will hit kfree(ERR_PTR(err))
and it will not be happy about it.

Signed-off-by: Mariusz Kozlowski <m.kozlowski@tuxland.pl>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Acked-by: Eric Van Hensbergen <ericvh@gmail.com>