fs/reiserfs: move prototype declaration to header file

Move prototype declaration to header file reiserfs/reiserfs.h from
reiserfs/super.c because they are used by more than one file.

This eliminates the following warning in reiserfs/bitmap.c:

  fs/reiserfs/bitmap.c:647:6: warning: no previous prototype for `show_alloc_options' [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Acked-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>