generic: add test for executables on read-only DAX mounts

generic: add test for executables on read-only DAX mounts

This adds a regression test for the following kernel patch:

commit 42d4a99b09cb ("ext4: fix fault handling when mounted with -o
dax,ro")

The above patch fixes an issue with ext4 where executables cannot be
run on read-only filesystems mounted with the DAX option.

This issue does not appear to be present in ext2 or XFS, as they
both pass the test.  I've also confirmed outside of the test that
they are both indeed able to execute binaries on read-only DAX
mounts.

Thanks to Randy Dodgen for the bug report and reproduction steps.

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: Randy Dodgen <rdodgen@gmail.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>