Add helper to calc dev extent length for chunk

chunk.length is the amount of real user data we can store in this chunk.

When we want to know how much raw disk space is occupied by a btrfs
chunk, we need to know how many device extents are used for it, and how
big they are (they are all the same size).

The amount can be found by looking at the amount of "stripe" objects
which are stored inside the "chunk" metadata object. The size of them is
not stored in there, but can be calculated using some simple rules, so
that we don't have to go and look up dev extent objects from another
tree.

btrfs/utils.py

@@ -127,6 +127,21 @@ def block_group_profile_str(flags):
     return block_group_flags_str(flags & BLOCK_GROUP_PROFILE_MASK)
 
 
+_chunk_to_dev_extent_length_fnmap = {
+    BLOCK_GROUP_SINGLE: lambda chunk: chunk.length,
+    BLOCK_GROUP_RAID0: lambda chunk: chunk.length // len(chunk.stripes),
+    BLOCK_GROUP_RAID1: lambda chunk: chunk.length,
+    BLOCK_GROUP_DUP: lambda chunk: chunk.length,
+    BLOCK_GROUP_RAID10: lambda chunk: chunk.length // (len(chunk.stripes) // 2),
+    BLOCK_GROUP_RAID5: lambda chunk: chunk.length // len(chunk.stripes - 1),
+    BLOCK_GROUP_RAID6: lambda chunk: chunk.length // len(chunk.stripes - 2),
+}
+
+
+def chunk_to_dev_extent_length(chunk):
+    return _chunk_to_dev_extent_length_fnmap[chunk.type & BLOCK_GROUP_PROFILE_MASK](chunk)
+
+
 _block_group_profile_ratio_map = {
     BLOCK_GROUP_SINGLE: 1,
     BLOCK_GROUP_RAID0: 1,