scripts/tags.sh: Add Page flag function magic

It takes a while to find the macro-magically defined Page*()
functions defined in include/linux/page-flags.h if you're new to
the kernel. Add some magic to the tags script to transform these
macros into the actual functions they are, so that tag jumping in
the mm code is a bit easier.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>