f2fs: rework fault injection handling to avoid a warning
When CONFIG_F2FS_FAULT_INJECTION is disabled, we get a warning about an
unused label:
fs/f2fs/segment.c: In function '__submit_discard_cmd':
fs/f2fs/segment.c:1059:1: error: label 'submit' defined but not used [-Werror=unused-label]
This could be fixed by adding another #ifdef around it, but the more
reliable way of doing this seems to be to remove the other #ifdefs
where that is easily possible.
By defining time_to_inject() as a trivial stub, most of the checks for
CONFIG_F2FS_FAULT_INJECTION can go away. This also leads to nicer
formatting of the code.