21214: Include project/collection singletons in customfs MemorySize.

21214: Include project/collection singletons in customfs MemorySize.

Previously, a hardlink returned its target's MemorySize, which
overreported the size of a tree in which a collection or project was
reachable by multiple paths.

Since 3d5a798ef6f4bd3b1a771bacdf0acf70edf6c1f5 a hardlink returns a
MemorySize reflecting only the hardlink itself, so the MemorySize of
the hardlink targets (project/collection singletons) needs to be added
explicitly by customFilesystem.

Arvados-DCO-1.1-Signed-off-by: Tom Clegg <tom@curii.com>