e2image: pass the correct size write_header

e2image: pass the correct size write_header

Commit bf0449b1a654, which added the ability to write qcow2 files,
generalized the write_header() file to take the size of the header
structure which it writes out.  Unfortunately, it changed the call
which supported original e2image format to pass in fs->blocksize,
instead of the actual size of the e2image header structure (which is
substantially smaller than fs->blocksize).  This meant that we copied
in stack garbage into the e2image file, and it made valgrind quite
unhappy.

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