Correctly parse -R flag arguments

Currently, only the 'b' flag takes an argument which is an offset into
the block at which a blkptr should be decoded. The index into the flag
string needed to be updated after parsing an argument.

Signed-off-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4304