bcachefs: New io_misc.c helpers

This pulls the non vfs specific parts of truncate and finsert/fcollapse
out of fs-io.c, and moves them to io_misc.c.

This is prep work for logging these operations, to make them atomic in
the event of a crash.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
5 files changed