LinuxKPI: add kstrtoint_from_user() and DECLARE_FLEX_ARRAY()

Add an implementation of kstrtoint_from_user() based on the other
implementations and an attempt at DECLARE_FLEX_ARRAY() which works
for the driver needing it.

Reviewed by:	hselasky
Differential Revision: https://reviews.freebsd.org/D34231

(cherry picked from commit c840d5c)

sys/compat/linuxkpi/common/include/linux/kernel.h

@@ -509,6 +509,21 @@ kstrtobool_from_user(const char __user *s, size_t count, bool *res)
 	return (kstrtobool(buf, res));
 }
 
+static inline int
+kstrtoint_from_user(const char __user *s, size_t count, unsigned int base,
+    int *p)
+{
+	char buf[36] = {};
+
+	if (count > (sizeof(buf) - 1))
+		count = (sizeof(buf) - 1);
+
+	if (copy_from_user(buf, s, count))
+		return (-EFAULT);
+
+	return (kstrtoint(buf, base, p));
+}
+
 static inline int
 kstrtou8_from_user(const char __user *s, size_t count, unsigned int base,
     u8 *p)
@@ -684,6 +699,9 @@ hex2bin(uint8_t *bindst, const char *hexsrc, size_t binlen)
 	return (0);
 }
 
+#define	DECLARE_FLEX_ARRAY(_t, _n)					\
+    struct { struct { } __dummy_ ## _n; _t _n[]; }
+
 /*
  * Checking if an option is defined would be easy if we could do CPP inside CPP.
  * The defined case whether -Dxxx or -Dxxx=1 are easy to deal with.  In either