ext4: introduce a mount option for iomap buffered I/O path

Since the iomap buffered I/O path does not yet support all existing ext4
features, it cannot be enabled by default. Introduce the
'buffered_iomap' and 'nobuffered_iomap' mount options to explicitly
enable or disable the iomap buffered I/O path for regular files.

Toggling this option via remount is allowed. The change of I/O path will
not take effect immediately. It will be deferred. The new setting will
only take effect after the inode is re-initialized (i.e., after the last
reference is dropped and the inode is re-read from disk).

Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Tested-by: syzbot@syzkaller.appspotmail.com
Link: https://patch.msgid.link/20261009103831.2978767-3-yi.zhang@huaweicloud.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>