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: Latchesar Ionkov <lucho@ionkov.net>
Cc: Eric Van Hensbergen <ericvh@ericvh.myip.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>