fs/minix: bugfix, number of indirect block ptrs per block depends on block size

The MINIX filesystem driver used a constant number of indirect block
pointers in an indirect block. This worked only for filesystems with 1kb
block, while the MINIX default block size is now 4kb. As a consequence,
large files were read incorrectly on such filesystems and writing a
large file would cause the filesystem to become corrupted. This patch
computes the number of indirect block pointers based on the block size,
making the driver work for each block size.

I would like to thank Feiran Zheng ('Fam') for pointing out the cause
of the corruption.

Signed-off-by: Erik van der Kouwe <vdkouwe@cs.vu.nl>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>