MFC r324007, r324012, r324014

r324007:
mountd: Replace malloc+strcpy to strdup

Reviewed by: bapt
Sponsored by: Gandi.net
Differential Revision: https://reviews.freebsd.org/D12503

r324012:
mountd: Remove unneeded cast

Reported by: kib
X MFC With: r324007

r324014:
mountd: Avoid memory leak by freeing dp_dirp

Introduced in r324007, the data alloced by strdup was never free'ed.
While here, remove cast to caddr_t when freeing dp.

Reported by: bde
X MFC With: r324007