Btrfs: PAGE_CACHE_SIZE is already unsigned long

PAGE_CACHE_SIZE == PAGE_SIZE is "unsigned long" everywhere, so there's no
need to cast it to "unsigned long".

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>