btrfs: remove the snapshot check from check_committed_ref()

btrfs: remove the snapshot check from check_committed_ref()

At check_committed_ref() we have this check to see if the data extent was
created in a generation lower than or equals to the generation where the
last snapshot for the root was created, and if so we return immediately
with 1, since it's very likely the extent is shared, referenced by other
root.

The only call chain for check_committed_ref() is the following:

   can_nocow_file_extent()
      btrfs_cross_ref_exist()
         check_committed_ref()

And we already do that snapshot check at can_nocow_file_extent(), before
we call btrfs_cross_ref_exist(). This makes the check done at
check_committed_ref() redundant, so remove it.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>