generic/312: fix dev_blocks calculation

generic/312: fix dev_blocks calculation

Current calculation fails when there are many same type of devices
with the testing device, eg sda1 sda2 ... sda10, sda11, sda12 ..
$(grep sda1 /proc/partitions) gets multiple numbers.

$ grep ram1 /proc/partitions
   1        1   10485760 ram1
   1       10   10485760 ram10
   1       11   10485760 ram11
   1       12   10485760 ram12
   1       13   10485760 ram13
   1       14   10485760 ram14
   1       15   10485760 ram15
$ grep -w ram1 /proc/partitions
   1        1   10485760 ram1

Fix this by adding the -w option to grep to match the block device
exactly.

Signed-off-by: Xiong Zhou <xzhou@redhat.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>