e2fsck: declare the size of bh->b_data to be 4096 in jfs_user.h

e2fsck: declare the size of bh->b_data to be 4096 in jfs_user.h

When allocating buffer_heads in e2fsck and debugfs the actual size of
the memory which is requested is based on the file system block size.
So the actual size of b_data in struct buffer_head doesn't actually
matter, except that it can triggers a UBSAN error when running the
e2fsck regression test.  So change it to be 4096 to avoid this false
positive.

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