ide: eliminate warnings in ide-tape.c

drivers/ide/ide-tape.c: In function '__idetape_kmalloc_stage':
drivers/ide/ide-tape.c:2588: warning: large integer implicitly truncated to unsigned type
drivers/ide/ide-tape.c:2616: warning: large integer implicitly truncated to unsigned type

b_size in struct idetape_bh is an unsigned short.  We sometimes assigne
PAGE_SIZE to it and PAGE_SIZE can be 64K or larger, so make it a u32.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>