Given a number of places in the tree that need to calculate this value
explicitly, might as well just create a macro for it.

(akpm: must be implemented as a macro for callee typeof() usage)
Signed-off-by: NRobert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>