Merge patch series "hide ->i_state behind accessors"

Mateusz Guzik <mjguzik@gmail.com> says:

Open-coded accesses prevent asserting they are done correctly. One
obvious aspect is locking, but significantly more can checked. For
example it can be detected when the code is clearing flags which are
already missing, or is setting flags when it is illegal (e.g., I_FREEING
when ->i_count > 0).

In order to keep things manageable this patchset merely gets the thing
off the ground with only lockdep checks baked in.

Current consumers can be trivially converted.

Suppose flags I_A and I_B are to be handled.

If ->i_lock is held, then:

state = inode->i_state          => state = inode_state_read(inode)
inode->i_state |= (I_A | I_B)   => inode_state_set(inode, I_A | I_B)
inode->i_state &= ~(I_A | I_B)  => inode_state_clear(inode, I_A | I_B)
inode->i_state = I_A | I_B      => inode_state_assign(inode, I_A | I_B)

If ->i_lock is not held or only held conditionally:

state = inode->i_state          => state = inode_state_read_once(inode)
inode->i_state |= (I_A | I_B)   => inode_state_set_raw(inode, I_A | I_B)
inode->i_state &= ~(I_A | I_B)  => inode_state_clear_raw(inode, I_A | I_B)
inode->i_state = I_A | I_B      => inode_state_assign_raw(inode, I_A | I_B)

The "_once" vs "_raw" discrepancy stems from the read variant differing
by READ_ONCE as opposed to just lockdep checks.

Finally, if you want to atomically clear flags and set new ones, the
following:

state = inode->i_state;
state &= ~I_A;
state |= I_B;
inode->i_state = state;

turns into:

inode_state_replace(inode, I_A, I_B);

* patches from https://lore.kernel.org/20251009075929.1203950-1-mjguzik@gmail.com:
  fs: make plain ->i_state access fail to compile
  xfs: use the new ->i_state accessors
  nilfs2: use the new ->i_state accessors
  overlayfs: use the new ->i_state accessors
  gfs2: use the new ->i_state accessors
  f2fs: use the new ->i_state accessors
  smb: use the new ->i_state accessors
  ceph: use the new ->i_state accessors
  btrfs: use the new ->i_state accessors
  Manual conversion to use ->i_state accessors of all places not covered by coccinelle
  Coccinelle-based conversion to use ->i_state accessors
  fs: provide accessors for ->i_state
  fs: spell out fenced ->i_state accesses with explicit smp_wmb/smp_rmb
  fs: move wait_on_inode() from writeback.h to fs.h

Signed-off-by: Christian Brauner <brauner@kernel.org>