fsdax: factor out a dax_fault_actor() helper

The core logic in the two dax page fault functions is similar. So, move
the logic into a common helper function. Also, to facilitate the
addition of new features, such as CoW, switch-case is no longer used to
handle different iomap types.

Signed-off-by: Shiyang Ruan <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Reviewed-by: Ritesh Harjani <[email protected]>
Reviewed-by: Darrick J. Wong <[email protected]>
Signed-off-by: Christoph Hellwig <[email protected]>
Signed-off-by: Darrick J. Wong <[email protected]>

Author: irides