geom_part: Fix potential integer overflow when checking size of the table

hdr_entries and hdr_entsz are both uint32_t as defined in UEFI spec.
Current spec does not have upper limit of the number of partition
entries and the size of partition entry, it is potential that malicious
or corrupted GPT header read from untrusted source contains large size of
entry number or size.

PR: 266548
Reviewed by: oshogbo, cem, imp, markj
Approved by: kp (mentor)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D36709