lib: Export errname

errname() returns the name of an errcode; this functionality is
otherwise only available for error pointers via %pE - bcachefs uses this
for better error messages.
Signed-off-by: Christopher James Halse Rogers <raof@ubuntu.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

lib/errname.c

@@ -228,3 +228,4 @@ const char *errname(int err)
 
 	return err > 0 ? name + 1 : name;
 }
+EXPORT_SYMBOL(errname);