Implements the optimization noted in commit f75fdf22 ("fuse: don't
use ->d_time"), as the additional memory can be significant.  (In
particular, on SLAB configurations this 8-byte alloc becomes 32 bytes).
Per-dentry, this can consume significant memory.
Reviewed-by: NShakeel Butt <shakeelb@google.com>
Signed-off-by: NKhazhismel Kumykov <khazhy@google.com>
Signed-off-by: NMiklos Szeredi <mszeredi@redhat.com>