Replace any '\' char with '\\' in table specification on input.

Replace any '\' char with '\\' in table specification on input.

Device-mapper in kernel uses '\' as escape character so it's better
to double it to avoid any confusion when using existing device names
with '\' in the table specification.

For example:

dmsetup create x --table "0 8 linear /dev/mapper/a\x20b 0"

should pass just fine now without a need to explicitly escape the '\' char
like this:

dmsetup create x --table "0 8 linear /dev/mapper/a\\x20b 0"