e2fsck: check for xattr value size integer wraparound

e2fsck: check for xattr value size integer wraparound

When checking an extended attrbiute block for correctness, we check if
the starting offset plus the value size exceeds the end of the block.
However, we weren't checking if the size was too large, and if it is
so large that it triggers a wraparound when we added the starting
offset, we won't notice the problem.  Add the missing check.

Reported-by: Nils Bars <nils.bars@rub.de>
Reported-by: Moritz Schlögel <moritz.schloegel@rub.de>
Reported-by: Nico Schiller <nico.schiller@rub.de>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>