fs/quota: add kernel_quotactl() helper; remove in-kernel call to syscall

Using the fs-internal kernel_quotactl() helper allows us to get rid of
the fs-internal call to the sys_quotactl() syscall.

This patch is part of a series which removes in-kernel calls to syscalls.
On this basis, the syscall entry path can be streamlined. For details, see
http://lkml.kernel.org/r/20180325162527.GA17492@light.dominikbrodowski.net

Acked-by: Jan Kara <jack@suse.cz>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>
diff --git a/fs/quota/quota.c b/fs/quota/quota.c
index 43612e2..860bfbe 100644
--- a/fs/quota/quota.c
+++ b/fs/quota/quota.c
@@ -833,8 +833,8 @@ static struct super_block *quotactl_block(const char __user *special, int cmd)
  * calls. Maybe we need to add the process quotas etc. in the future,
  * but we probably should use rlimits for that.
  */
-SYSCALL_DEFINE4(quotactl, unsigned int, cmd, const char __user *, special,
-		qid_t, id, void __user *, addr)
+int kernel_quotactl(unsigned int cmd, const char __user *special,
+		    qid_t id, void __user *addr)
 {
 	uint cmds, type;
 	struct super_block *sb = NULL;
@@ -885,3 +885,9 @@ SYSCALL_DEFINE4(quotactl, unsigned int, cmd, const char __user *, special,
 		path_put(pathp);
 	return ret;
 }
+
+SYSCALL_DEFINE4(quotactl, unsigned int, cmd, const char __user *, special,
+		qid_t, id, void __user *, addr)
+{
+	return kernel_quotactl(cmd, special, id, addr);
+}