9p: fix bad error path in conversion routines

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>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>