xfs: don't allocate COW extents when unsharing a hole

It doesn't make sense to allocate a COW extent when unsharing a hole
because holes cannot be shared.

Fixes: 1f1397b7 ("xfs: don't allocate into the data fork for an unshare request")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Link: https://lore.kernel.org/r/172796813277.1131942.5486112889531210260.stgit@frogsfrogsfrogs


Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>