iomap: Clean up __generic_write_end calling

Move the call to __generic_write_end into iomap_write_end instead of
duplicating it in each of the three branches.  This requires open coding
the generic_write_end for the buffer_head case.

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