Take into account when zpool history block grows exceeding 128KB in zpool(8)
and zdb(8) by growing the buffer on demand with a cap of 1GB (specified in
spa_history_create_obj()).

PR:		bin/186574
Submitted by:	Andrew Childs <lorne cons org nz> (with changes)
MFC after:	2 weeks

(cherry picked from commit 78e547c5404320fbd04ad333c9f9691a30b0e86e)

Ticket: #4736
(cherry picked from commit 887098cab7b481404f44107ec9fac6e0a65d4ade)