e2fsck: Fix potential data corruptor bug in journal recovery

e2fsck: Fix potential data corruptor bug in journal recovery

While synchronizing e2fsck's recovery.c with the latest 2.6 kernel
sources, I discovered a serious bug that apparently had been fixed in
the kernel sometime between Deceber 2003 and April 2005, but which had
not been carried over to e2fsprogs.  Specifically, when blocks whose
first 4 bytes are JFS_MAGIC_NUMBER (0xc03b3998) are written into the
journal, the first 4 bytes zero'ed out.  A one character typo meant
that when the blocks were replayed by e2fsck, the JFS_MAGIC_NUMBER
would not be restored.

Oops.

Fortunately, it is *highly* unlikely that ext4 metadata blocks will
contain that magic number in the first four bytes, and data=journalled
is a relatively rarely used.

This commit fixes this bug, as well as updating e2fsck's recovery.c to
be in sync with 2.6.25.

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