f2fs: add lookup_mode mount option

For casefolded directories, f2fs may fall back to a linear search if
a hash-based lookup fails. This can cause severe performance
regressions.

While this behavior can be controlled by userspace tools (e.g. mkfs,
fsck) by setting an on-disk flag, a kernel-level solution is needed
to guarantee the lookup behavior regardless of the on-disk state.

This commit introduces the 'lookup_mode' mount option to provide this
kernel-side control.

The option accepts three values:
- perf: (Default) Enforces a hash-only lookup. The linear fallback
  is always disabled.
- compat: Enables the linear search fallback for compatibility with
  directory entries from older kernels.
- auto: Determines the mode based on the on-disk flag, preserving the
  userspace-based behavior.

Signed-off-by: Daniel Lee <chullee@google.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>