libext2fs: check EA block headers when reading in the block

libext2fs: check EA block headers when reading in the block

When reading an EA block in from disk, do a quick sanity check of the
block header, and return an error if we think we have garbage.  Teach
e2fsck to ignore the new error code in favor of doing its own
checking, and remove the strict_csums bits while we're at it.

(Also document some assumptions in the new ext_attr code.)

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>