Btrfs: fix data allocation hint start

Sometimes our start allocation hint when we cow a file can be either
EXTENT_HOLE or some other such place holder, which is not optimal.  So if we
find that our em->block_start is one of these special values, check to see
where the first block of the inode is stored, and use that as a hint.  If that
block is also a special value, just fallback on a hint of 0 and let the
allocator figure out a good place to put the data.

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