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>

module/spl/spl-kmem.c

@@ -843,9 +843,6 @@ kv_alloc(spl_kmem_cache_t *skc, int size, int flags)
 	if (skc->skc_flags & KMC_KMEM) {
 		ptr = (void *)__get_free_pages(flags, get_order(size));
 	} else {
-#ifdef HAVE_PMD_ALLOC_WITH_MASK
-		ptr = __vmalloc(size, flags|__GFP_HIGHMEM, PAGE_KERNEL);
-#else
 		/*
 		 * As part of vmalloc() an __pte_alloc_kernel() allocation
 		 * may occur.  This internal allocation does not honor the
@@ -869,7 +866,6 @@ kv_alloc(spl_kmem_cache_t *skc, int size, int flags)
 		} else {
 			ptr = __vmalloc(size, flags|__GFP_HIGHMEM, PAGE_KERNEL);
 		}
-#endif
 	}
 
 	/* Resulting allocated memory will be page aligned */