xfs: add xfs_atomic_write_cow_iomap_begin()

For CoW-based atomic writes, reuse the infrastructure for reflink CoW fork
support.

Add ->iomap_begin() callback xfs_atomic_write_cow_iomap_begin() to create
staging mappings in the CoW fork for atomic write updates.

The general steps in the function are as follows:
- find extent mapping in the CoW fork for the FS block range being written
	- if part or full extent is found, proceed to process found extent
	- if no extent found, map in new blocks to the CoW fork
- convert unwritten blocks in extent if required
- update iomap extent mapping and return

The bulk of this function is quite similar to the processing in
xfs_reflink_allocate_cow(), where we try to find an extent mapping; if
none exists, then allocate a new extent in the CoW fork, convert unwritten
blocks, and return a mapping.

Performance testing has shown the XFS_ILOCK_EXCL locking to be quite
a bottleneck, so this is an area which could be optimised in future.

Christoph Hellwig contributed almost all of the code in
xfs_atomic_write_cow_iomap_begin().

Reviewed-by: Darrick J. Wong <djwong@kernel.org>
[djwong: add a new xfs_can_sw_atomic_write to convey intent better]
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: John Garry <john.g.garry@oracle.com>