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>

include/linux/quotaops.h

@@ -27,6 +27,9 @@ static inline bool is_quota_modification(struct inode *inode, struct iattr *ia)
 		(ia->ia_valid & ATTR_GID && !gid_eq(ia->ia_gid, inode->i_gid));
 }
 
+int kernel_quotactl(unsigned int cmd, const char __user *special,
+		    qid_t id, void __user *addr);
+
 #if defined(CONFIG_QUOTA)
 
 #define quota_error(sb, fmt, args...) \