floppy: fix for PAGE_SIZE != 4KB

For years I wondered why the floppy driver does not just work on
sparc64, e.g:

root@SUNW_375_0066:# disktype /dev/fd0
disktype: Can't open /dev/fd0: No such device or address

[ 525.341906] disktype: attempt to access beyond end of device
fd0: rw=0, sector=0, nr_sectors = 16 limit=8
[ 525.341991] floppy: error 10 while reading block 0

Turns out floppy.c __floppy_read_block_0 tries to read one page for
the first test read to determine the disk size and thus fails if that
is greater than 4k. Adjust minimum MAX_DISK_SIZE to PAGE_SIZE to fix
floppy on sparc64 and likely all other PAGE_SIZE != 4KB configs.

Cc: [e-mail]
Signed-off-by: René Rebe <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

drivers/block/floppy.c

···
  * This default is used whenever the current disk size is unknown.
  * [Now it is rather a minimum]
  */
-#define MAX_DISK_SIZE 4		/* 3984 */
+#define MAX_DISK_SIZE (PAGE_SIZE / 1024)
 
 /*
  * globals used by 'result()'