Btrfs: check return value of btrfs_search_slot properly

Doing an audit of where we use btrfs_search_slot only showed one place where we
don't check the return value of btrfs_search_slot properly.  Just fix
mark_extent_written to see if btrfs_search_slot failed and act accordingly.
Thanks,

Signed-off-by: Josef Bacik <josef@redhat.com>