bcachefs: Rework fiemap transaction restart handling

Restart handling in the previous patch was incorrect, so: move btree
operations into a separate helper, and run it with a lockrestart_do().

Additionally, clarify whether pagecache or the btree takes precedence.

Right now, the btree takes precedence: this is incorrect, but it's
needed to pass fstests. Add a giant comment explaining why.

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