bcachefs: bch2_fs_emergency_read_only2()

bcachefs: bch2_fs_emergency_read_only2()

More error message cleanup: instead of multiple printk()s per error, we
want to be building up a single error message in a printbuf, so that it
can be printed with indenting that shows grouping and avoid errors
getting interspersed or lost in the log.

This gets rid of most calls to bch2_fs_emergency_read_only(). We still
have calls to
 - bch2_fatal_error()
 - bch2_fs_fatal_error()
 - bch2_fs_fatal_err_on()

that need work.

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