bcachefs: Log truncate operations

bcachefs: Log truncate operations

Previously, we guaranteed atomicity of truncate after unclean shutdown
with the BCH_INODE_I_SIZE_DIRTY flag - which required a full scan of the
inodes btree.

Recently the deleted inodes btree was added so that we no longer have to
scan for deleted inodes, but truncate was unfinished and that change
left it broken.

This patch uses the new logged operations btree to fix truncate
atomicity; we now log an operation that can be replayed at the start of
a truncate.

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