Revert "Detect kernels that honor gfp flags passed to vmalloc()"

This reverts commit 36811b4430.
Which is no longer required because there is now SPL code in
place to safely handle the deadlocks the kernel patch was designed
to address.  Therefore we can unconditionally use vmalloc() and
drop all the PF_MEMALLOC code.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

config/spl-build.m4

@@ -88,7 +88,6 @@ AC_DEFUN([SPL_AC_CONFIG_KERNEL], [
 	SPL_AC_2ARGS_ZLIB_DEFLATE_WORKSPACESIZE
 	SPL_AC_SHRINK_CONTROL_STRUCT
 	SPL_AC_RWSEM_SPINLOCK_IS_RAW
-	SPL_AC_PMD_ALLOC_WITH_MASK
 ])
 
 AC_DEFUN([SPL_AC_MODULE_SYMVERS], [
@@ -2235,38 +2234,3 @@ AC_DEFUN([SPL_AC_RWSEM_SPINLOCK_IS_RAW], [
 	])
 	EXTRA_KCFLAGS="$tmp_flags"
 ])
-
-dnl #
-dnl # Proposed VM Subsystem Bug Fix
-dnl # https://bugs.gentoo.org/show_bug.cgi?id=416685
-dnl #
-dnl # Make __pte_alloc_kernel() honor gfp flags passed to vmalloc()
-dnl # This is detected by checking a macro that is changed to support this.
-dnl #
-AC_DEFUN([SPL_AC_PMD_ALLOC_WITH_MASK], [
-	AC_MSG_CHECKING([whether pmd_alloc_with_mask exists])
-	SPL_LINUX_TRY_COMPILE([
-		#if !defined(CONFIG_MMU)
-		#define CONFIG_MMU
-		#endif
-
-		#if defined(RCH_HAS_4LEVEL_HACK)
-		#undef RCH_HAS_4LEVEL_HACK
-		#endif
-
-		#include <linux/mm.h>
-	],[
-		struct mm_struct init_mm;
-		pud_t *pud = NULL;
-		unsigned long addr = 0;
-		gfp_t gfp_mask = GFP_KERNEL;
-
-		pmd_alloc_with_mask(&init_mm, pud, addr, gfp_mask);
-	],[
-		AC_MSG_RESULT(yes)
-		AC_DEFINE(HAVE_PMD_ALLOC_WITH_MASK, 1,
-		          [pmd_alloc_with_mask exists])
-	],[
-		AC_MSG_RESULT(no)
-	])
-])