bcachefs: Assorted checkpatch fixes

bcachefs: Assorted checkpatch fixes

checkpatch.pl gives lots of warnings that we don't want - suggested
ignore list:

 ASSIGN_IN_IF
 UNSPECIFIED_INT	- bcachefs coding style prefers single token type names
 NEW_TYPEDEFS		- typedefs are occasionally good
 FUNCTION_ARGUMENTS	- we prefer to look at functions in .c files
			  (hopefully with docbook documentation), not .h
			  file prototypes
 MULTISTATEMENT_MACRO_USE_DO_WHILE
			- we have _many_ x-macros and other macros where
			  we can't do this

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