For reasons which escape me, blockdev_get_blocks() is returning -EIO if the
mapping request straddles the end of the disk.  This causes reads which
straddle end-of-disk to return -EIO.

Fix it up so that it correctly trims the returned result down to match the
size of the device, for both reads and writes.

If a write attempt is fully beyond end-of-device then return -EIO.  We never
get this far anyway, because generic_write_checks() will return -ENOSPC if all
of the write lies outside a blockdev's i_size.
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>