vfs: reduce failing trylock attempts in mnt_vnode_next_active

[mjg]

Summary

When doing poudriere -j 104, after 4 minutes of tracing:

kernel`mnt_vnode_next_active                        _mtx_trylock_flags_int                                            1          1683089
kernel`mnt_vnode_next_active                        _mtx_trylock_flags_int                                            0          4781020

That is, majority of attempts to trylock has failed.

[kib]

This cannot go in without explanation why mtx_owner is checked, in the comment above. But it should not go in regardless, if such check is useful perhaps it should be added to VI_TRYLOCK, or mtx_trylock, or another version of VI_TRYLOCK created.

[mjg]

• add VI_TRYLOCK_CONTENDED

[mjg]

With filtering support (D22690) this should not be needed