bcachefs: Compression levels

This allows including a compression level when specifying a compression
type, e.g.
  compression=zstd:15

Values from 1 through 15 indicate compression levels, 0 or unspecified
indicates the default.

For LZ4, values 3-15 specify that the HC algorithm should be used.

Note that for compatibility, extents themselves only include the
compression type, not the compression level. This means that specifying
the same compression algorithm but different compression levels for the
compression and background_compression options will have no effect.

XXX: perhaps we could add a warning for this

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>