direct-io: add a hook for the fs to provide its own submit_bio function

Because BTRFS can do RAID and such, we need our own submit hook so we can setup
the bio's in the correct fashion, and handle checksum errors properly.  So there
are a few changes here

1) The submit_io hook.  This is straightforward, just call this instead of
submit_bio.

2) Allow the fs to return -ENOTBLK for reads.  Usually this has only worked for
writes, since writes can fallback onto buffered IO.  But BTRFS needs the option
of falling back on buffered IO if it encounters a compressed extent, since we
need to read the entire extent in and decompress it.  So if we get -ENOTBLK back
from get_block we'll return back and fallback on buffered just like the write
case.

I've tested these changes with fsx and everything seems to work.  Thanks,

Signed-off-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>