akpm and wli each discovered unfortunate behaviour of dbench on tmpfs:
after tmpfs has reached its data memory limit, dbench continues to
lseek and write, and tmpfs carries on allocating unlimited metadata
blocks to accommodate the data it then refuses.  That particular
behaviour could be simply fixed by checking earlier; but I think tmpfs
metablocks should be subject to the memory limit, and included in df
and du accounting.  Also, manipulate inode->i_blocks under lock, was
missed before.