bcachefs: Errcode tracepoint, documentation

Add a tracepoint for downcasting private errors to standard errors, so
they can be recovered even when not logged; also, add some
documentation.

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

bcachefs: Fix W=12 build errors

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

bcachefs: Break up io.c

More reorganization, this splits up io.c into
- io_read.c
- io_misc.c - fallocate, fpunch, truncate
- io_write.c

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

bcachefs: More style fixes

Fixes for various checkpatch errors.

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

bcachefs: Errcodes can now subtype standard error codes

The next patch is going to be adding private error codes for all the
places we return -ENOSPC.

Additionally, this patch updates return paths at all module boundaries
to call bch2_err_class(), to return the standard error code.

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

bcachefs: Improved errcodes

Instead of overloading standard error codes (EINTR/EAGAIN), and defining
short lists of error codes in multiple places that potentially end up
overlapping & conflicting, we're now going to have one master list of
error codes.

Error codes are defined with an x-macro: thus we also have
bch2_err_str() now.

Also, error codes have a class field. Now, instead of checking for
errors with ==, code should use bch2_err_matches(), which returns true
if the error is equal to or a sub-error of the error class.

This means we can define unique errors for every source location where
an error is generated, which will help improve our error messages.

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