kernel: new mount cache

this is a new more simple version of the mount cache
that does not require dynamic allocations for extends.

the Mntcache structure now contains a page bitmap
that is used for quick page invalidation. the size
of the bitmap is proportional to MAXCACHE.

instead of keeping track of cached range in the
Extend data structure, we keep all the information
in the Page itself. the offset from the page where
the cache range starts is in the low PGSHIT bits and
the end in the top bits of Page.va.

we choose Page.daddr to map 1:1 the Mountcache number
and page number (pn) in the Mountcache. to find a page,
we first check the bitmap if the page is there and then
do a pagelookup() with the daddr key.

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