btrfs: use PAGE_{ALIGN, ALIGNED, ALIGN_DOWN} macro

The header file linux/mm.h provides PAGE_ALIGN, PAGE_ALIGNED,
PAGE_ALIGN_DOWN macros. Use these macros to make code more
concise.

Signed-off-by: Yushan Zhou <katrinzhou@tencent.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>