tarfs: Implement VOP_BMAP

[markj]

Summary

This lets tarfs provide readahead/behind hints to the VFS, which helps
memory-mapped I/O performance, important when running faulting in
executables out of a tarfs mount as one might if tarfs is used to back
the root filesystem, for instance. This is particularly noticeable when
the backing tarball is zstd-compressed.

The implementation simply returns the extent of the virtual block
containing the target offset, clamped by the maximum I/O size. This is
perhaps simplistic; it effectively just chooses values that would
correspond to a single VOP_READ call in tarfs_read_file().

Sponsored by: Klara, Inc.

[kib]

sys/fs/tarfs/tarfs_vnops.c

147

Why not spell the type as u_unt?

172

Can we move the a_bnp calculation before the loop? Then I do not see the loop is needed if both a_runp and a_runb are NULL.

[markj]

Apply feedback from kib.

[kib]

BTW we never call bmap with runp NULL and runb non-NULL? Perhaps a debugpre hook for VOP_BMAP stating this would be due.

[markj]

In D44626#1017623, @kib wrote:

BTW we never call bmap with runp NULL and runb non-NULL? Perhaps a debugpre hook for VOP_BMAP stating this would be due.

UFS makes this assumption too, and indeed I do not see any such callers.