Workaround for Linux PowerPC GPL-only cpu_has_feature()

Linux since 4.7 makes interface 'cpu_has_feature' to use jump labels on
powerpc if CONFIG_JUMP_LABEL_FEATURE_CHECKS is enabled, in this case
however the inline function references GPL-only symbol
'cpu_feature_keys'.

ZFS currently uses 'cpu_has_feature' either directly or indirectly from
several places; while it is unknown how this issue didn't break ZFS on
64-bit little-endian powerpc, it is known to break ZFS with many Linux
versions on both 32-bit and 64-bit big-endian powerpc.

Until this issue is fixed in Linux, we have to workaround it by
overriding affected inline functions without depending on
'cpu_feature_keys'.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: WHR <msl0000023508@gmail.com>
Closes #14590

config/kernel.m4

@@ -151,6 +151,12 @@ AC_DEFUN([ZFS_AC_KERNEL_TEST_SRC], [
 	ZFS_AC_KERNEL_SRC_IDMAP_MNT_API
 	ZFS_AC_KERNEL_SRC_IATTR_VFSID
 	ZFS_AC_KERNEL_SRC_FILEMAP
+	case "$host_cpu" in
+		powerpc*)
+			ZFS_AC_KERNEL_SRC_CPU_HAS_FEATURE
+			ZFS_AC_KERNEL_SRC_FLUSH_DCACHE_PAGE
+			;;
+	esac
 
 	AC_MSG_CHECKING([for available kernel interfaces])
 	ZFS_LINUX_TEST_COMPILE_ALL([kabi])
@@ -275,6 +281,12 @@ AC_DEFUN([ZFS_AC_KERNEL_TEST_RESULT], [
 	ZFS_AC_KERNEL_IDMAP_MNT_API
 	ZFS_AC_KERNEL_IATTR_VFSID
 	ZFS_AC_KERNEL_FILEMAP
+	case "$host_cpu" in
+		powerpc*)
+			ZFS_AC_KERNEL_CPU_HAS_FEATURE
+			ZFS_AC_KERNEL_FLUSH_DCACHE_PAGE
+			;;
+	esac
 ])
 
 dnl #