bfd: Implement buffered reads

The restore times look like

Before patch:
	  futex:      370  3.554482 (84.2%)
	 umount:       41  0.234796 (5.6%)
	   read:     4737  0.113987 (2.7%)
	recvmsg:       43  0.100083 (2.4%)
	  wait4:       10  0.033344 (0.8%)

After patch:
	  futex:      187  1.547642 (72.9%)
	 umount:       41  0.234595 (11.0%)
	recvmsg:       43  0.075738 (3.6%)
	  flock:       42  0.038696 (1.8%)
	  clone:       35  0.037699 (1.8%)

Most of the time we wait for other processes to restore,
but that's OK (would only affect parallel restore). And
we see that read-s really go away (onto 7th position).

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>