Caveats with compression and fallocate

[jimis]

No description provided.

[jimis]

ping @adam900710

[adam900710]

Although I have merged the two patches, they all went through some modification.
If you want to contribute to btrfs in the future, it would be better to follow the next two points:

• Write a commit message
  Even for simple doc update, please write a short description for the change.

• Use a "Signed-off-by" line
  This can be generated by "-s" option of "git commit".