device be created read+write, check to see if the backing store is read only
through the use of the access(2) system call. If this check fails returning
EACCES, EPERM or EROFS then gracefully downgrade the access to read only. Also
print a warning message to stderr, informing the user that the access mode
they requested is not available.
This behavior used to be handled by md(4) but was changed in revision 1.154
Discussed with: pjd, phk, Dario Freni <saturnero at freesbie dot org>
Reviewed by: phk
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