e2fsck: try to salvage extent blocks with bad checksums

e2fsck: try to salvage extent blocks with bad checksums

Remove the code that would zap an extent block immediately if the
checksum failed (i.e. strict_csums).  Instead, we'll only do that if
the extent block header shows obvious structural problems; if the
header checks out, then we'll iterate the block and see if we can
recover some extents.

Requires a minor modification to ext2fs_extent_get such that the
extent block will be returned in the buffer even if the return code
indicates a checksum error.  This brings its behavior in line with
the rest of libext2fs.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>