uprobes: Kill the unnecesary filp != NULL check in __copy_insn()

__copy_insn(filp) can only be called after valid_vma() returns T,
vma->vm_file passed as "filp" can not be NULL.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Anton Arapov <anton@redhat.com>
Acked-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>

kernel/events/uprobes.c

@@ -501,9 +501,6 @@ __copy_insn(struct address_space *mapping, struct file *filp, char *insn,
 {
 	struct page *page;
 
-	if (!filp)
-		return -EINVAL;
-
 	if (!mapping->a_ops->readpage)
 		return -EIO;
 	/*