MFC r335559:

Don't bail out when we find primary and secondary bootblocks miscompare.
We do not have code to fix this situation, and the mismatch does not
prevent the kernel driver from consuming the file system, and some factory
formatted SD cards seem to have a garbage backup block.

This makes the code match to its comments (replacing pfatal with pwarn).

Inspired by: NetBSD r1.13
Inspired by: https://android.googlesource.com/platform/external/fsck_msdos/+/b47b16353f3db228711dded9f7c975b820059ddc