ext4: recover from filesystem corruption when reading

Some fixes when reading EXT files and directory entries were identified
after using e2fuzz to corrupt an EXT3 filesystem:

 - Stop reading directory entries if the offset becomes badly aligned.

 - Avoid overwriting memory by clamping the length used to zero the buffer
   in ext4fs_read_file.  Also sanity check blocksize.

Signed-off-by: Ian Ray <ian.ray@ge.com>
Signed-off-by: Martyn Welch <martyn.welch@collabora.co.uk>
Reviewed-by: Stefano Babic <sbabic@denx.de>