autoconf: refactor linux-checks.m4

autoconf: refactor linux-checks.m4

Further refactoring of the autoconf macros. Divy up the linux kernel
checks into smaller files.

This is a non-functional change. Care has been taken preserve the
ordering of the autoconf tests. Except for whitespace, the generated
configure file has not been changed by this refactoring.  This has been
verified with a 'diff -u -w -B' comparison of the generated configure
file before and after applying this commit.

Reviewed-on: https://gerrit.openafs.org/12844
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit 6a2b85cd4c00a08e165cb96d2cb56bf87c6324bc)

Change-Id: Iae325bc14fb160f27791b2f3d82198fe671badd8
Reviewed-on: https://gerrit.openafs.org/12878
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>