btrfs-progs: Fix one-byte overlap bug in free_block_group_cache

btrfs-progs: Fix one-byte overlap bug in free_block_group_cache

free_block_group_cache() calls clear_extent_bits() with wrong end, which
is one byte larger than the correct range.

This will cause the next adjacent cache state to be split.  And due to
the split, private pointer (which points to block group cache) will be
reset to NULL.

This is very hard to detect as this function only gets called in
cleanup_temp_chunks() which is just before mkfs finishes.  This bug only
gets exposed when reworking --rootdir option.

Signed-off-by: Qu Wenruo <quwenruo.btrfs@gmx.com>
Signed-off-by: David Sterba <dsterba@suse.com>