shared lock instead of exclusive in set_rootvnode

hi

in set_rootvnode() there is 

p->p_fd->fd_cdir = rootvnode;
p->p_fd->fd_rdir = rootvnode;

protected just by FILEDESC_SLOCK(), I dont think this is correct
although it might be benign as this is used only during boot.

still, if it's not correct it should be fixed

comments?

roman