Remove NPTL_GUARD_WITHIN_STACK

Commit 4b2f65b increased the user
space stack by 4x to resolve certain stack overflows.  As such it
no longer makes sense to worry about a single extra page which
might or might not be part of the process stack.  There is now
ample headroom for normal usage.

By eliminating this configure check we are also resolving the
following segfault which intentionally occurs at configure time
and may be logged in dmesg.

  conftest[22156]: segfault at 7fbf18a47e48 ip 00000000004007fe
  sp 00007fbf18a4be50 error 6 in conftest[400000+1000]

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

config/user.m4

@@ -10,5 +10,4 @@ AC_DEFUN([ZFS_AC_CONFIG_USER], [
 	ZFS_AC_CONFIG_USER_LIBBLKID
 	ZFS_AC_CONFIG_USER_LIBSELINUX
 	ZFS_AC_CONFIG_USER_FRAME_LARGER_THAN
-	ZFS_AC_CONFIG_USER_STACK_GUARD
 ])

include/sys/zfs_context.h

@@ -190,12 +190,6 @@ extern void vpanic(const char *, __va_list);
 #define	STACK_SIZE		24576	/* Solaris */
 #endif
 
-#ifdef NPTL_GUARD_WITHIN_STACK
-#define	EXTRA_GUARD_BYTES	PAGESIZE
-#else
-#define	EXTRA_GUARD_BYTES	0
-#endif
-
 /* in libzpool, p0 exists only to have its address taken */
 typedef struct proc {
 	uintptr_t	this_is_never_used_dont_dereference_it;

lib/libzpool/kernel.c

@@ -175,8 +175,7 @@ zk_thread_create(caddr_t stk, size_t stksize, thread_func_t func, void *arg,
 	 * on Linux.
 	 */
 
-	stack = PTHREAD_STACK_MIN + MAX(stksize, STACK_SIZE) * 4 +
-			EXTRA_GUARD_BYTES;
+	stack = PTHREAD_STACK_MIN + MAX(stksize, STACK_SIZE) * 4;
 
 	VERIFY3S(pthread_attr_init(&attr), ==, 0);
 	VERIFY3S(pthread_attr_setstacksize(&attr, stack), ==, 0);