Fixed invalid resource re-use in file_find()

File descriptors are a per-process resource. The same descriptor
in different processes can refer to different files. find_file()
incorrectly assumed that file descriptors are globally unique.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes ZFS issue #386

include/sys/vnode.h

@@ -158,6 +158,7 @@ typedef struct vnode {
 
 typedef struct vn_file {
 	int		f_fd;		/* linux fd for lookup */
+	struct task_struct *f_task;	/* linux task this fd belongs to */
 	struct file	*f_file;	/* linux file struct */
 	atomic_t	f_ref;		/* ref count */
 	kmutex_t	f_lock;		/* struct lock */