xfs: upgrade the extent counters in xfs_reflink_end_cow_extent later

Defer the extent counter size upgrade until we know we're going to
modify the extent mapping.  This also defers dirtying the transaction
and will allow us safely back out later in the function in later
changes.

Fixes: 4f86bb4b ("xfs: Conditionally upgrade existing inodes to use large extent counters")
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>