ext4: partially enable iomap for the buffered I/O path of regular files

Introduce ext4_enable_buffered_iomap() to determine whether a regular
file inode should use the iomap buffered I/O path. We now support the
default filesystem features, mount options, and the bigalloc feature.
However, inline data, fsverity, fscrypt, indirect inode type, and
data=journal mode are not fully supported.

The decision is made at inode initialization time in __ext4_new_inode()
and __ext4_iget() by setting the EXT4_STATE_BUFFERED_IOMAP state flag.
If any of these unsupported features are met, the inode silently falls
back to the traditional buffer_head path. Switching the buffered I/O
path on an active inode is not supported, with the exception of changing
a per-inode journal flag.

For features like encryption, verity, and inline data that can be
dynamically enabled at the superblock level, checking the global feature
flag avoids the complexity of toggling the path on individual inodes.

Additionally:

 - Extend ext4_inode_journal_mode() to force ordered mode for inodes
   using the iomap path under a data=journal mount. For the global data
   journal mode (EXT4_MOUNT_JOURNAL_DATA), dynamic enablement is
   deferred until the next inode re-initialization. For the per-inode
   data journal mode (EXT4_INODE_JOURNAL_DATA), dynamic changes take
   effect immediately, as it is safe to switch address_space operations
   and drop all page cache under i_rwsem and filemap_invalidate_lock.

 - Add a WARN_ON_ONCE() guard in _ext4_get_block() to catch inodes
   using the iomap path from accidentally entering the legacy
   buffer_head writeback path.

 - Place silent BUFFERED_IOMAP checks in ext4_do_writepages() and
   ext4_iomap_writepages() under the writepages rwsem. Unlike
   _ext4_get_block(), the writeback path does not hold i_rwsem or
   invalidate_lock and can race with ext4_change_inode_journal_flag()
   switching the inode's a_ops.

 - Reject extent-to-indirect migration via ext4_ind_migrate() for inodes
   on the iomap path.

Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Link: https://patch.msgid.link/20260903124017.2325538-2-yi.zhang@huaweicloud.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>