fs/coredump.c: log if a core dump is aborted due to changed file permissions

For obvious security reasons, a core dump is aborted if the filesystem
cannot preserve ownership or permissions of the dump file.

This affects filesystems like e.g.  vfat, but also something like a 9pfs
share in a Qemu test setup, running as a regular user, depending on the
security model used.  In those cases, the result is an empty core file and
a confused user.

To hopefully save other people a lot of time figuring out the cause, this
patch adds a simple log message for those specific cases.

[akpm@linux-foundation.org: s/|%s/%s/ in printk text]

Link: https://lkml.kernel.org/r/20210701233151.102720-1-david.oberhollenzer@sigma-star.at
Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>