Resolve the mount point's path with realpath(2) before checking if file system is mounted. This prevevents duplicated mounts.
Resolve the mount point's path with realpath(2) before checking if file
system is mounted. This prevevents duplicated mounts.

The change I made against the original patch is to fall back to the given
path on realpath(2) failure instead of exiting with an error.

Submitted by:	Andreas Kohn <andreas@syndrom23.de>
PR:		bin/89782
MFC after:	3 days

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