vfs: add vfs_get_fsid() helper

Wrapper around statfs() interface.

Cc: Al Viro <[e-mail]>
Signed-off-by: Amir Goldstein <[e-mail]>
Signed-off-by: Jan Kara <[e-mail]>

fs/statfs.c

···
 	return retval;
 }
 
+int vfs_get_fsid(struct dentry *dentry, __kernel_fsid_t *fsid)
+{
+	struct kstatfs st;
+	int error;
+
+	error = statfs_by_dentry(dentry, &st);
+	if (error)
+		return error;
+
+	*fsid = st.f_fsid;
+	return 0;
+}
+EXPORT_SYMBOL(vfs_get_fsid);
+
 int vfs_statfs(const struct path *path, struct kstatfs *buf)
 {
 	int error;

include/linux/statfs.h

···
 #define ST_NODIRATIME	0x0800	/* do not update directory access times */
 #define ST_RELATIME	0x1000	/* update atime relative to mtime/ctime */
 
+struct dentry;
+extern int vfs_get_fsid(struct dentry *dentry, __kernel_fsid_t *fsid);
+
 #endif