fbdev: Avoid file argument in fb_pgprotect()

Only PowerPC's fb_pgprotect() needs the file argument, although
the implementation in either phys_mem_access_prot() or
pci_phys_mem_access_prot() does not use it. Pass NULL to the internal
helper in preparation of further updates. A later patch will remove
the file parameter from fb_pgprotect().

While at it, replace the shift operation with PHYS_PFN().

v5:
* state function names in commit description (Javier)

Suggested-by: Christophe Leroy <[e-mail]>
Signed-off-by: Thomas Zimmermann <[e-mail]>
Reviewed-by: Arnd Bergmann <[e-mail]>
Reviewed-by: Javier Martinez Canillas <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

arch/powerpc/include/asm/fb.h

···
 static inline void fb_pgprotect(struct file *file, struct vm_area_struct *vma,
 				unsigned long off)
 {
-	vma->vm_page_prot = phys_mem_access_prot(file, off >> PAGE_SHIFT,
+	/*
+	 * PowerPC's implementation of phys_mem_access_prot() does
+	 * not use the file argument. Set it to NULL in preparation
+	 * of later updates to the interface.
+	 */
+	vma->vm_page_prot = phys_mem_access_prot(NULL, PHYS_PFN(off),
 						 vma->vm_end - vma->vm_start,
 						 vma->vm_page_prot);
 }