Convert a missed u_char cpu -> int cpu.

This was caught by a gcc build.

Reported by:	luigi
Sponsored by:	Norse Corp, Inc.
Provide vfs suspension support only for filesystems which need it.

Need is expressed by providing vfs_susp_clean function in vfsops.

Differential Revision:	D952
Reviewed by:	kib (previous version)
MFC after:	2 weeks