Two fixes for pgcache read.

[kib]

Summary

1. vn_read_from_obj(): there is no reason why vp->v_object cannot be NULL. If it is, it's fine, handle it by delegating to VOP_READ().
2. nullfs: provide custom bypass for VOP_READ_PGCACHE(). Normal bypass expects locked vnode. Ensure liveness of the lower vnode by taking the upper vnode interlock, which is also taked by null_reclaim() when setting v_data to NULL.

Reported and tested by: pho

[mjg]

sys/kern/vfs_vnops.c

874–876

probably would be best to atomic_load_ptr all cases like this

[kib]

Use atomic_load_ptr().

[mjg]

Committed in f7af6e5e54ebd1e3d70dd3d9d9660f049c1dbd2b