fs: fold __generic_write_end back into generic_write_end

This effectively reverts a6d639da ("fs: factor out a
__generic_write_end helper") as we now open code what is left of that
helper in iomap.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>