A tmpfs user reported increasingly slow directory reads when repeatedly
creating and unlinking in a mkstemp-like way.  The negative dentries
accumulate alarmingly (until memory pressure finally frees them), and are
just a hindrance to any in-memory filesystem.  simple_lookup set d_op to
arrange for negative dentries to be deleted immediately.

(But I failed to discover how it is that on-disk filesystems seem to keep
their negative dentries within manageable bounds: this effect was gross
with tmpfs or ramfs, but no problem at all with extN or reiser.)
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>