e2fsck: check a file system mounted read-only if forced

e2fsck: check a file system mounted read-only if forced

Previously e2fsck would only allow a mounted file system to be checked
if it was the root file system and it was mounted read-only.  Now
allow any file system mounted read-only if the -f option is specified.

This makes it easier to test how e2fsck handles checking file systems
which are mounted without having to test on the root file system.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>