Use reallocf instead of malloc to fix leak with outbuf_pmap

The previous code overwrote outbuf_pmap's memory with malloc once per
loop iteration, which leaked its memory; use reallocf instead to ensure
that memory is properly free'd each loop iteration.

Add a outbuf_pmap = NULL in the failure case to avoid a double-free
at the bottom of the function.

Differential Revision: https://reviews.freebsd.org/D6495
MFC after: 1 week
Reported by: Coverity
CID: 1038776
Reviewed by: markj, pfgj
Sponsored by: EMC / Isilon Storage Division