journald: Retry if posix_fallocate returned -1 (EINTR)

On some conditions (particularly when mobile CPUs are going to sleep),
the posix_fallocate(), which is called when a new journal file is allocated,
can return -1 (EINTR). This is counted as a fatal error. So the journald
closes both old and journals, and simply throwing away further incoming
events, because of no log files open.

Introduce posix_fallocate_loop() that restarts the function in the case
of EINTR. Also let's make code base more uniform by returning negative
values on error.

Fix assert in test-sigbus.c that incorrectly counted positive values as
success. After changing the function return values, that will actually work.

Fixes: #19041

Signed-off-by: Igor Zhbanov <i.zhbanov@omprussia.ru>