nilfs2: fix gcc unused-but-set-variable warnings

Fix the following build warnings:

 $ make W=1
 [...]
   CC [M]  fs/nilfs2/btree.o
 fs/nilfs2/btree.c: In function 'nilfs_btree_split':
 fs/nilfs2/btree.c:923:8: warning: variable 'newptr' set but not used [-Wunused-but-set-variable]
   __u64 newptr;
         ^
 fs/nilfs2/btree.c:922:8: warning: variable 'newkey' set but not used [-Wunused-but-set-variable]
   __u64 newkey;
         ^
   CC [M]  fs/nilfs2/dat.o
 fs/nilfs2/dat.c: In function 'nilfs_dat_prepare_end':
 fs/nilfs2/dat.c:158:8: warning: variable 'start' set but not used [-Wunused-but-set-variable]
   __u64 start;
         ^
   CC [M]  fs/nilfs2/segment.o
 fs/nilfs2/segment.c: In function 'nilfs_segctor_do_immediate_flush':
 fs/nilfs2/segment.c:2433:6: warning: variable 'err' set but not used [-Wunused-but-set-variable]
   int err;
       ^
   CC [M]  fs/nilfs2/sufile.o
 fs/nilfs2/sufile.c: In function 'nilfs_sufile_alloc':
 fs/nilfs2/sufile.c:320:27: warning: variable 'ncleansegs' set but not used [-Wunused-but-set-variable]
   unsigned long nsegments, ncleansegs, nsus, cnt;
                            ^
   CC [M]  fs/nilfs2/alloc.o
 fs/nilfs2/alloc.c: In function 'nilfs_palloc_prepare_alloc_entry':
 fs/nilfs2/alloc.c:478:38: warning: variable 'groups_per_desc_block' set but not used [-Wunused-but-set-variable]
   unsigned long n, entries_per_group, groups_per_desc_block;
                                       ^

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/fs/nilfs2/sufile.c b/fs/nilfs2/sufile.c
index 7ff8f15..52821ff 100644
--- a/fs/nilfs2/sufile.c
+++ b/fs/nilfs2/sufile.c
@@ -319,7 +319,7 @@
 	size_t susz = NILFS_MDT(sufile)->mi_entry_size;
 	__u64 segnum, maxsegnum, last_alloc;
 	void *kaddr;
-	unsigned long nsegments, ncleansegs, nsus, cnt;
+	unsigned long nsegments, nsus, cnt;
 	int ret, j;
 
 	down_write(&NILFS_MDT(sufile)->mi_sem);
@@ -329,7 +329,6 @@
 		goto out_sem;
 	kaddr = kmap_atomic(header_bh->b_page);
 	header = kaddr + bh_offset(header_bh);
-	ncleansegs = le64_to_cpu(header->sh_ncleansegs);
 	last_alloc = le64_to_cpu(header->sh_last_alloc);
 	kunmap_atomic(kaddr);