Fixed a few minor nits in liblp

Bug: 122616553
Test: built and ran liblp_test on Pixel 3 with Q weekly build

Added missing __attribute__((packed)) in two metadata structures.
Fixed error logging message when repairing primary metadata.
Few very minor additions related to metadata validation.
Fixed an off by one error in the validation of partition name length.

Change-Id: Ic777baf97871c786a209da7c32bc13c1360a8482
Signed-off-by: Ramon Pantin <pantin@google.com>