Move statistics keeping from afs_dcache to afs_fetchstore.

Move statistics keeping from afs_dcache to afs_fetchstore.

More context of the afs_CacheFetchProc() call in GetDCache is now inlined
in the afs_CacheFetchProc. Boosts readability of GetDCache and saves
stack, as counter pointers need not be passed anymore. The statistics
code is more readable due to lesser indentation.

Reviewed-on: http://gerrit.openafs.org/115
Reviewed-by: Russ Allbery <rra@stanford.edu>
Tested-by: Russ Allbery <rra@stanford.edu>