Replace check for _POSIX_MEMLOCK w/ HAVE_MLOCKALL

zed supports a '-M' cmdline opt to lock all pages in memory via
mlockall().  The _POSIX_MEMLOCK define is checked to determine whether
this function is supported.  The current test assumes mlockall()
is supported if _POSIX_MEMLOCK is non-zero.  However, this test is
insufficient according to mlock(2) and sysconf(3).  If _POSIX_MEMLOCK
is -1, mlockall() is not supported; but if _POSIX_MEMLOCK is 0,
availability must be checked at runtime.

This commit adds an autoconf check for mlockall() to user.m4.  The zed
code block for mlockall() is now guarded with a test for HAVE_MLOCKALL.
If defined, mlockall() will be called and its runtime availability
checked via its return value.

Signed-off-by: Chris Dunlap <cdunlap@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #2

cmd/zed/zed.c

@@ -97,10 +97,7 @@ _setup_sig_handlers(void)
 static void
 _lock_memory(void)
 {
-#if ! _POSIX_MEMLOCK
-	zed_log_die("Failed to lock memory pages: mlockall() not supported");
-
-#else /* _POSIX_MEMLOCK */
+#if HAVE_MLOCKALL
 	int i = 0;
 	const int max_tries = 10;
 
@@ -114,7 +111,9 @@ _lock_memory(void)
 	}
 	zed_log_die("Failed to lock memory pages: %s", strerror(errno));
 
-#endif /* _POSIX_MEMLOCK */
+#else /* HAVE_MLOCKALL */
+	zed_log_die("Failed to lock memory pages: mlockall() not supported");
+#endif /* HAVE_MLOCKALL */
 }
 
 /*

config/user.m4

@@ -13,4 +13,7 @@ AC_DEFUN([ZFS_AC_CONFIG_USER], [
 	ZFS_AC_CONFIG_USER_LIBBLKID
 	ZFS_AC_CONFIG_USER_FRAME_LARGER_THAN
 	ZFS_AC_CONFIG_USER_RUNSTATEDIR
+dnl #
+dnl #	Checks for library functions
+	AC_CHECK_FUNCS([mlockall])
 ])