Make the ramdisk blocksize configurable at kernel compilation time rather
than only at boot or module load time, like a couple of the other ramdisk
options.  I found this handy awhile back but thought little of it, until
recently asked by a few of the testing folks here to be able to do the same
thing for their automated test setups.

The Kconfig comment is largely lifted from comments in rd.c, and hopefully
this will increase the chances of making folks aware that the default value
often isn't a great choice here (for increasing values of PAGE_SIZE, even
moreso).

Signed-off-by: Nathan Scott <nathans@sgi.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>