lsfd: use the list of block devices in /proc/devices for decoding SOURCE column

lsfd: use the list of block devices in /proc/devices for decoding SOURCE column

For decoding the SOURCE column of a fd opening a block devices, the
origina code uses /proc/partitions only. However, this is not enough
for decoding /dev/nullb0. Though is is a block device node, the block
device behind the node is not listed in /proc/partitions.

This change uses the information min /proc/devices as the fallback of
/proc/partitions.

Signed-off-by: Masatake YAMATO <yamato@redhat.com>