geom_linux_lvm: Check the offset of physical volume header

The LVM label is stored on any of the first four sectors, and the
PV (physical volume) header is stored within the same sector following
the LVM label. The current implementation does not fully check the
offset of PV header, when attaching a bad formatted LVM PV the kernel
may crash due to out-of-bounds memory read.

PR: 266562
Reviewed by: jhb
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D36773