coredump: treat RLIMIT_CORE below page size as disabling coredumps (#3932)

coredump: treat RLIMIT_CORE below page size as disabling coredumps (#3932)

The kernel treats values below a certain threshold (minfmt->min_coredump
which is initialized do ELF_EXEC_PAGESIZE, which varies between architectures,
but is usually the same as PAGE_SIZE) as disabling coredumps [1].
Any core image below ELF_EXEC_PAGESIZE will yield an invalid backtrace anyway [2],
so follow the kernel and not try to parse or store such images.

[1] https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/fs/coredump.c#n660
[2] systemd-coredump[16260]: Process 16258 (sleep) of user 1002 dumped core.
                                Stack trace of thread 16258:
                                #0  0x00007f1d8b3d3810 n/a (n/a)

https://bugzilla.redhat.com/show_bug.cgi?id=1309172#c19