generic: test overwriting file with mmap on a full filesystem

generic: test overwriting file with mmap on a full filesystem

Test that overwriting a file with mmap when the filesystem has no more
space available for data allocation works. The motivation here is to check
that NOCOW mode of a COW filesystem (such as btrfs) works as expected.

This currently fails with btrfs but it's fixed by a kernel patch that has
the subject:

   btrfs: fix -ENOSPC mmap write failure on NOCOW files/extents

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>