use lazy umount when umount returns EBUSY
use lazy umount when umount returns EBUSY

When the umount fails, we force the umount and make the mount point
unaccessible by using a lazy umount.

Signed-off-by: Daniel Lezcano <3211518dd4d87cb3f7ee12cb52e14b7fb2bc0446@fr.ibm.com>

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