libext2fs: set the fs block size to new_io in ext2fs_rewrite_to_io()

libext2fs: set the fs block size to new_io in ext2fs_rewrite_to_io()

e2image manually opens a new IO channel, and then sets the file system
to use this new IO channel via ext2fs_rewrite+to_io().  We need to
make sure the IO channel is set to the file system's block size to
avoid some nasty buffer overruns.

[ Modified by tytso to use io_channel_set_blksize() ]

Signed-off-by: Kit Westneat <kwestneat@ddn.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>