Since the size of GPT entry may differ from the sizeof(struct gpt_ent),
use the size from GPT header to iterate entries.

Suggested by:	marcel@
MFC after:	1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@270521 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f