[memfs] Refine the max page limit on a memfs

When a memfs is created without an explicit max page limit, the fs
assumes UINT64_MAX, which is confusing when manifested in
QueryFilesystem calls. This patch refines the cap to the amount of
available physical memory.

Test: /boot/test/fs/memfs-test
Change-Id: I10fada0b8b2eb7b33f270cd31c9d859157475c1d