libext2fs: return an error when byte swapping a corrupted dirblock block

libext2fs: return an error when byte swapping a corrupted dirblock block

Except for e2fsck (where we want to expose the corrupted directory
entries to e2fsck mostly so that the e2fsck output stays the same on
big-endian machines compared to little-endian machines, so we don't
break our regression tests), if the directory block is corrupted, and
ext2fs_dirent_swab_in[2](), trips across this, return an error.  This
will make sure that naive users of libextfs will not try to handle a
corrupted directory block.  This prevents potential buffer overruns in
the byte swapping code paths.

This commit does not cause any functional change on little-endian
systems.

Addresses-Coverity-Bug: 1433408
Signed-off-by: Theodore Ts'o <tytso@mit.edu>