tmpfs: Move partial page invalidation to a separate helper

[khng]

Summary

The partial page invalidation code is factored out to be a separate
helper from tmpfs_reg_resize().

Sponsored by: The FreeBSD Foundation

[kib]

sys/fs/tmpfs/tmpfs_subr.c

404

Replace the whole 'else' branch with something along the lines:

vm_page_free(m);
m = NULL;
if (!ignerr)
    error = EIO;

and change return (0); at the end with return (error);

1755

By convention 'rv' var holds a Mach error code, not errno. Since tmpfs_partial_page_invalidate() returns errno, better rename rv there.

[khng]

Cleanups suggested by kib@