At find_delalloc_subrange(), when we need to get the next extent map, we
do a full search on the extent map tree (a red black tree). This is fine
but it's a lot more efficient to simply use rb_next(), which typically
requires iterating over less nodes of the tree and never needs to compare
the ranges of nodes with the one we are looking for.

So add a public helper to extent_map.{h,c} to get the extent map that
immediately follows another extent map, using rb_next(), and use that
helper at find_delalloc_subrange().
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>