Btrfs: check for duplicate iov_base's when doing dio reads

Apparently it is ok to submit a read to an IDE device with the same target page
for different offsets.  This is what Windows does under qemu.  The problem is
under DIO we expect them to be different buffers for checksumming reasons, and
so this sort of thing will result in checksum errors, when in reality the file
is fine.  So when reading, check to make sure that all iov bases are different,
and if they aren't fall back to buffered mode, since that will work out right.
Thanks,

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