e2fsck: check and fix tails of all bitmap blocks

e2fsck: check and fix tails of all bitmap blocks

Currently, e2fsck effectively checks only tail of the last inode and
block bitmap in the filesystem. Thus if some previous bitmap has unset
bits it goes unnoticed.  Mostly these tail bits in the bitmap are
ignored; however, if blocks_per_group are smaller than 8*blocksize,
the multi-block allocator in the kernel can get confused when the tail
bits are unset and return bogus free extent.

Add support to libext2fs to check these bitmap tails when loading
bitmaps (as that's about the only place which has access to the bitmap
tail bits) and make e2fsck use this functionality to detect buggy bitmap
tails and fix them (by rewriting the bitmaps).

Reported-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>