extfs: rpm: use printf instead of echo, to calculate DESCRIPTION length.

extfs: rpm: use printf instead of echo, to calculate DESCRIPTION length.

Out test input intentionally contains weird characters in the DESCRIPTION field
(see the file 'test.spec'[1]).

It turns out that backslash sequences, as in our DESCRIPTION field, are
processed by the 'echo' command in some shells (i.e., Dash). In other words,
'echo' can't be used to print arbitrary data verbatim. Indeed, the standard
says[2] that "if any of the operands contain a <backslash> character, the
results are implementation-defined". You can see the problem by running:

    $ echo "one \\a two"

under both Bash and Dash: you'll get different results.

The solution: we replace 'echo' with 'printf'.

[1] https://midnight-commander.org/browser/tests/src/vfs/extfs/helpers-list/misc/rpm/test.spec#L34
[2] http://pubs.opengroup.org/onlinepubs/9699919799/utilities/echo.html

Signed-off-by: Mooffie <mooffie@gmail.com>