Use sscanf instead of atoi when parsing e2fsck options

Use sscanf instead of atoi when parsing e2fsck options

atoi() does not check for errors so it shouldn't be used for human
input.  For example, if the user enters the command "e2fsck -C -n" and
forgets that -C requires an argument, the -n will be used as the
argument to -C, and not parsed as an option.  When using sscanf(),
this error case can be detected.

Addresses-Debian-Bug: #435381

Signed-off-by: Bernd Schubert <bs@q-leap.de>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>