Implement buffer_boundary() for ext3.

buffer_boundary() is an I/O scheduling hint which the filesystem's
get_block() function passes up to the BIO assembly code.  It is
described in fs/mpage.c

The time to read 1,000 52 kbyte files goes from 8.6 seconds down to 2.9
seconds.  52 kbytes is the worst-case size.