iomap: iomap_write_end cleanup

In iomap_write_end(), only call iomap_write_failed() on the byte range
that has failed.  This should improve code readability, but doesn't fix
an actual bug because iomap_write_failed() is called after updating the
file size here and it only affects the memory beyond the end of the
file.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>