Provide vfs suspension support only for filesystems which need it.

[mjg]

Summary

Instead of moving vn_start_write and friends to vfsops, I decided to add a
vfsconf flag instead.

This speeds up some operations which otherwise contend a lot on mp lock.

This business with mp's becoming NULL at any time is quite fishy (I do realize
they are not freed), I'll poke around holding them while there are any vnodes.

I'll poke around speeding up vn_start_write & friends later.

[kib]

Overall, I am fine with the change. I think there should be a comment somewhere that working suspension requires the VFCF flag.

Consider testing vfs_susp_clean for non-NULL instead of using the flag.

sys/kern/vfs_vnops.c

1575

This should have return type bool.

1576

The name is too long, even by my standards.

1579

!= 0);

1582

This should return bool too.

[mjg]

Take 2.

vfs_susp_clean is null in tmpfs.

I don't really see any place to put a comment about the flag. On the other hand
this patch adds relevant assertions to suspending functions, so this should be acceptable.

[kib]

My english is bad, but if I would select a name, I use vn_suspendable() or like.

WRT to tmpfs, I would set vfs_susp_clean to an empty stub.

I do not insist on either change above.