If PAGE_CACHE_SIZE were to differ from PAGE_SIZE, the VM_ACCT macro,
and shmem_nopage's vm_pgoff manipulation, were still not quite right.

Slip a cond_resched_lock into shmem_truncate's long loop; but not into
shmem_unuse_inode's, since other locks held, and swapoff awful anyway.

Move SetPageUptodate to where it's not already set.  Replace
copy_from_user by __copy_from_user since access already verified.

Replace BUG()s by BUG_ON()s.  Remove an uninteresting PAGE_BUG().