e2fsck: teach e2fsck how to deal with bigalloc in non-extent-mapped inodes

e2fsck: teach e2fsck how to deal with bigalloc in non-extent-mapped inodes

Currently the bigalloc implementation in the kernel requires extents,
but this restriction might get relaxed in the future.  Also, old
versions of mke2fs that supported bigalloc during early testing
created the root and lost+found directories without using
extent-mapped inodes.  This makes it possible for e2fsck to better
support these old legacy file systems if it comes across them.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>