Fix for issue: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=271105

The ext2fs does not support disks with sectorsize more 512 bytes. The main issue is in reading superblock, which is not aligned with 4k value.
Re-implement the superblock reading logic to make it indifferent to disk logical sector size. The logical sector size more then page size is not supported, like it is doing on Linux side.
Also, make some cleanups.

Tested using https://github.com/thedrgreenthumb/Jenkins/tree/master/Target/ext2fs_test_all testcases with md devices with logical sector sizes:
512, 1024, 2048, 4096