Add the b:airline_whitespace_disabled feature.

If the per-buffer variable b:airline_whitespace_disabled is defined
and non-zero, whitespace checking will be disabled for that buffer.

autoload/airline/extensions/whitespace.vim

@@ -51,6 +51,7 @@ endfunction
 
 function! airline#extensions#whitespace#check()
   if &readonly || !&modifiable || !s:enabled || line('$') > s:max_lines
+          \ || get(b:, 'airline_whitespace_disabled', 0)
     return ''
   endif
 

doc/airline.txt

@@ -502,6 +502,15 @@ eclim <https://eclim.org>
   matters for mix-indent-file algorithm: >
   let airline#extensions#c_like_langs = ['c', 'cpp', 'cuda', 'javascript', 'ld', 'php']
 <
+* disable whitespace checking for an individual buffer >
+  " Checking is enabled by default because b:airline_whitespace_disabled
+  " is by default not defined:
+  unlet b:airline_whitespace_disabled
+
+  " If b:airline_whitespace_disabled is defined and is non-zero for a buffer,
+  " then whitespace checking will be disabled for that buffer; for example:
+  " let b:airline_whitespace_disabled = 1
+<
 -------------------------------------                      *airline-tabline*
 Note: If you're using the ctrlspace tabline only the option marked with (c)
 are supported!