btrfs: add helper to replace extent map range with a new extent map

We have several places that need to drop all the extent maps in a given
file range and then add a new extent map for that range. Currently they
call btrfs_drop_extent_map_range() to delete all extent maps in the range
and then keep trying to add the new extent map in a loop that keeps
retrying while the insertion of the new extent map fails with -EEXIST.

So instead of repeating this logic, add a helper to extent_map.c that
does these steps and name it btrfs_replace_extent_map_range(). Also add
a comment about why the retry loop is necessary.

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