elf: Remove excessive p_align check on PT_LOAD segments [BZ #28688]

p_align does not have to be a multiple of the page size.  Only PT_LOAD
segment layout should be aligned to the page size.

1: Remove p_align check against the page size.
2. Use the page size, instead of p_align, to check PT_LOAD segment layout.

Reviewed-by: Florian Weimer <fweimer@redhat.com>