e2fsck: check extent-mapped directories with really large logical blocks

e2fsck: check extent-mapped directories with really large logical blocks

E2fsck was missing a check for directories with logical blocks so
large that i_size > 2GB.  Without this check the test image found in
the new test f_toobig_extent_dir will cause e2fsck to die with a
memory allocation failure:

Error storing directory block information (inode=12, block=0, num=475218819): Memory allocation failed
e2fsck: aborted

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reported-by: Andrey Melnikov <temnota.am@gmail.com>