Rename get_pv_header() and get_label_header() to *_from_buf()
Rename get_pv_header() and get_label_header() to *_from_buf()

For consistency with rest of the codebase.

Signed-off-by: Andy Grover <b7d524d2f5cc5aebadb6b92b08d3ab26911cde33@redhat.com>

agrover/melvin