[PATCH] constify inode accessors

Change the signature of i_size_read(), IMINOR() and IMAJOR() because they,
or the functions they call, will never modify the argument.

Signed-off-by: Jan Engelhardt <jengelh@gmx.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>