gfs2: No more gfs2_find_jhead caching

We are no longer calling gfs2_find_jhead() on the same log twice, so
there is no more reason for keeping the log contents cached across those
calls.  In addition, log head lookup and log header writing didn't go
through the same address space and so the caching wasn't even fully
working, anyway.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>