Unbreak makefs -t cd9660 after r289687

buffer_head needs to be freed -- not buffer

Detected by jemalloc, i.e. running makefs failed the arena assert
because my copy of malloc on CURRENT is compiled with the default
!MALLOC_PRODUCTION asserts on

Pointyhat to: ngie
PR: 203647
X-MFC with: r289687
Sponsored by: EMC / Isilon Storage Division