e2fsck: verify extent tree blocks and clear the bad ones

e2fsck: verify extent tree blocks and clear the bad ones

When we encounter an extent tree block that passes the header check
but fails the checksum, offer to clear just that extent block instead
of failing the whole tree, which results in the entire inode being
wiped out.

Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>