scst: Raise the minimum upstream kernel version to 3.18

Upstream Linux 3.11-3.17 uses older percpu_ref implementations whose
memory lifetime and reinitialization rules do not support the revival
needed when SCST cancels a suspend with outstanding commands. Maintaining
those implementations requires a separate compatibility path.

Require upstream Linux 3.18 or later and remove the eight 3.10-3.17
entries from the nightly matrix and the 3.10 selector from regression CI.
Keep every distribution entry, including EL7 7.5-7.9 and UEK4/UEK5. EL7
has the newer percpu_ref implementation despite its 3.10 base version.

Enforce the minimum and vendor exception in the shared build header.
This is a support-policy change, not an analyzer-warning fix. It does not
fix the remaining reinit-versus-resurrect mismatch on supported kernels.

.github/kernel-matrix.conf

@@ -28,7 +28,6 @@ GITHUB_REGRESSION_KERNEL_SELECTORS="\
 4.14 \
 4.9 \
 3.18 \
-3.10 \
 Rocky^10.2 \
 Rocky^10.1 \
 Rocky^9.8 \

nightly/conf/nightly.conf

@@ -70,14 +70,6 @@ ABT_KERNELS="  \
 4.0.9-nc       \
 3.19.8-nc      \
 3.18.140-nc    \
-3.17.8-nc      \
-3.16.85-nc     \
-3.15.10-nc     \
-3.14.79-nc     \
-3.13.11-nc     \
-3.12.74-nc     \
-3.11.10-nc     \
-3.10.108-nc    \
 6.12.0-211.66.1.el10_2^Rocky^10.2-nc     \
 6.12.0-124.56.1.el10_1^Rocky^10.1-nc     \
 5.14.0-687.58.1.el9_8^Rocky^9.8-nc       \

scst/include/backport.h

@@ -30,8 +30,9 @@
 #error RHEL 6 is no longer supported. Please upgrade to RHEL 7 or later.
 #endif
 
-#if LINUX_VERSION_CODE < KERNEL_VERSION(3, 10, 0)
-#error SCST does not support kernels older than 3.10.0.
+#if LINUX_VERSION_CODE < KERNEL_VERSION(3, 18, 0) && \
+	(!defined(RHEL_MAJOR) || RHEL_MAJOR - 0 < 7)
+#error SCST requires Linux 3.18 or later, except for supported RHEL 7 kernels.
 #endif
 
 #include <linux/bio.h>