libshare: nfs: always try to mkdir()

libshare: nfs: always try to mkdir()

This also works out to one syscall if the directory exists,
but is one syscall shorter if it doesn't.

Reviewed-by: Don Brady <don.brady@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #12067