Partially decompose priv_check by adding priv_check_cred_vfs_generation

During buildkernel there are very frequent calls to priv_check and they
all are for PRIV_VFS_GENERATION (coming from stat/fstat).

This results in branching on several potential privileges checking if
perhaps that's the one which has to be evaluated.

Instead of the kitchen-sink approach provide a way to have commonly used
privs directly evaluated.