nvdimm/namespace: Fix comment typo

The double `existing' is duplicated in the comment, remove one.

Signed-off-by: Jason Wang <wangborong@cdjrlc.com>
Link: https://lore.kernel.org/r/20220802201918.8408-1-wangborong@cdjrlc.com


Signed-off-by: Dan Williams <dan.j.williams@intel.com>