Allow setting of number of raw devices as a module parameter

Allow setting of maximal number of raw devices as a module parameter. This
requires changing of static array into a vmalloced one (the array is going to
be too large for kmalloc).

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>