btrfs: introduce a new shutdown state

btrfs: introduce a new shutdown state

A new fs state EMERGENCY_SHUTDOWN is introduced, which is btrfs'
equivalent of XFS_IOC_GOINGDOWN or EXT4_IOC_SHUTDOWN, after entering
emergency shutdown state, all operations will return errors (-EIO), and
can not be bring back to normal state until unmouont.

The new state will reject the following file operations:

- read_iter()
- write_iter()
- mmap()
- open()
- remap_file_range()
- uring_cmd()
- splice_read()
  This requires a small wrapper to do the extra shutdown check, then call
  the regular filemap_splice_read() function

This should reject most of the file operations on a shutdown btrfs.

And for the existing dirty folios, extra shutdown checks are introduced
to the following functions:

- run_delalloc_nocow()
- run_delalloc_compressed()
- cow_file_range()

So that dirty ranges will still be properly cleaned without being
submitted.

Finally the shutdown state will also set the fs error, so that no new
transaction will be committed, protecting the metadata from any possible
further corruption.

And when the fs entered shutdown mode for the first time, a critical
level kernel message will show up to indicate the incident.

That message will be important for end users as rejected delalloc ranges
will output error messages, hopefully that shutdown message and the fact
that all fs operations are returning error will prevent end users from
getting too confused about the delalloc error messages.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Anand Jain <asj@kernel.org>
Tested-by: Anand Jain <asj@kernel.org>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>