falloc vs reflink revisited

[Christoph Hellwig]

Hi Darrick,

I know that I suggested unsharing blocks on fallocate, but it turns out
this is causing problems.  Applications expect falloc to be a fast
metadata operation, and copying a potentially large number of blocks
is against that expextation.  This is especially bad for the NFS
server, which should not be blocked for a long time in a synchronous
operation.

I think we'll have to remove the unshare and just fail the fallocate
for a reflinked region for now.  I still think it makes sense to expose
an unshare operation, and we probably should make that anyother
fallocate mode.

Opininions?