Fixes a regression from the recent "remove ->get_blocks() support"

Fixes a regression from the recent "remove ->get_blocks() support"
change.  inode->i_blkbits should be used when making a get_block_t
request of a filesystem instead of dio->blkbits, as that does not
indicate the filesystem block size all the time (depends on request
alignment - see start of __blockdev_direct_IO).

Signed-off-by: Nathan Scott <nathans@sgi.com>
Acked-by: Badari Pulavarty <pbadari@us.ibm.com>