FAT: Remove some unused macros and #ifndef around the remaining ones.

Also adjust header paths order so the FAT compatibility headers
always come before system ones.

src/add-ons/kernel/file_systems/fat/Jamfile

@@ -1,12 +1,12 @@
 SubDir HAIKU_TOP src add-ons kernel file_systems fat ;
 
+local commonSource ;
+include [ FDirName $(HAIKU_TOP) src add-ons kernel file_systems fat JamCommon ] ;
+
 UsePrivateKernelHeaders ;
 UsePrivateHeaders shared storage file_systems ;
 UseHeaders [ FDirName $(HAIKU_TOP) headers compatibility bsd ] : true ;
 
-local commonSource ;
-include [ FDirName $(HAIKU_TOP) src add-ons kernel file_systems fat JamCommon ] ;
-
 KernelAddon fat :
 	$(commonSource)
 	mime_ext_table.c

src/add-ons/kernel/file_systems/fat/bsd/sys/param.h

@@ -71,11 +71,15 @@
 #define EJUSTRETURN (-2) /* don't modify regs, just return */
 
 /* Macros for counting and rounding. */
+#ifndef rounddown
 #define rounddown(x, y) (((x) / (y)) * (y))
-#define rounddown2(x, y) __align_down(x, y) /* if y is power of two */
+#endif
+#ifndef roundup
 #define roundup(x, y) ((((x) + ((y) -1)) / (y)) * (y)) /* to any y */
-#define roundup2(x, y) __align_up(x, y) /* if y is powers of two */
+#endif
+#ifndef powerof2
 #define powerof2(x) ((((x) -1) & (x)) == 0)
+#endif
 
 
 #endif // FAT_PARAM_H