xfs: set IOMAP_F_SHARED for all COW fork allocations

Change to always set xfs_buffered_write_iomap_begin for COW fork
allocations even if they don't overlap existing data fork extents,
which will allow the iomap_end callback to detect if it has to punch
stale delalloc blocks from the COW fork instead of the data fork.  It
also means we sample the sequence counter for both the data and the COW
fork when writing to the COW fork, which ensures we properly revalidate
when only COW fork changes happens.

This is essentially a revert of commit 72a048c1 ("xfs: only set
IOMAP_F_SHARED when providing a srcmap to a write"). This is fine because
the problem that the commit fixed has now been dealt with in iomap by
only looking at the actual srcmap and not the fallback to the write
iomap.

Note that the direct I/O path was never changed and has always set
IOMAP_F_SHARED for all COW fork allocations.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>