UBI: Fastmap: Wrap fastmap specific function in a ifdef

...such that we can implement NOP variants of some functions.
This will help to reduce fastmap specific ifdefs in other c files.
Signed-off-by: Richard Weinberger <richard@nod.at>
Reviewed-by: Tanya Brokhman <tlinder@codeaurora.org>

drivers/mtd/ubi/ubi.h

@@ -871,10 +871,14 @@ int ubi_compare_lebs(struct ubi_device *ubi, const struct ubi_ainf_peb *aeb,
 		      int pnum, const struct ubi_vid_hdr *vid_hdr);
 
 /* fastmap.c */
+#ifdef CONFIG_MTD_UBI_FASTMAP
 size_t ubi_calc_fm_size(struct ubi_device *ubi);
 int ubi_update_fastmap(struct ubi_device *ubi);
 int ubi_scan_fastmap(struct ubi_device *ubi, struct ubi_attach_info *ai,
 		     int fm_anchor);
+#else
+static inline int ubi_update_fastmap(struct ubi_device *ubi) { return 0; }
+#endif
 
 /* block.c */
 #ifdef CONFIG_MTD_UBI_BLOCK