atomics: Constify loads: Remove now superfluous uses of DECONST()

No functional change (intended).

Reviewed by: kib
MFC after: 4 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48497

(cherry picked from commit 9b7157c690723279d448fc7ea9c1f9374ad4511a)

During MFC, the change to:
'sys/compat/linuxkpi/common/include/linux/seqlock.h'
was elided as lkpi_seqprop_sequence() has never been MFCed to stable/13.