ceph: use GFP_NOFS for cap flush allocation in writeback path

ceph_alloc_cap_flush() is called from ceph_writepages_start() inside
the writeback layer, where other allocations in the same path
(ceph_osdc_alloc_request, ceph_osdc_alloc_messages) already use
GFP_NOFS.  A GFP_KERNEL allocation here can trigger direct reclaim
that recursively enters the filesystem writeback path:

  ceph_writepages_start()                       // inode A writeback
    ceph_alloc_cap_flush()
      kmem_cache_alloc(..., GFP_KERNEL)
        [direct reclaim]
          try_to_free_pages()
            shrink_slab()
              super_cache_scan()
                prune_icache_sb()
                  inode_lru_isolate()
                    iput() -> evict(inode_B)
                      [inode_B has dirty pages]
                      filemap_flush()
                        ceph_writepages_start()  // re-enters writeback
                          ceph_alloc_cap_flush()
                            -> RECURSION / STACK OVERFLOW

All 11 callers of ceph_alloc_cap_flush() are in write or writeback
contexts: writepages (x2), write_iter, fallocate, copy_file_range,
setxattr, setattr, and page_mkwrite.

Signed-off-by: Xiubo Li <xiubo.li@clyso.com>
Reviewed-by: Viacheslav Dubeyko <slava@dubeyko.com>
Reviewed-by: Alex Markuze <amarkuze@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

fs/ceph/caps.c

@@ -1850,7 +1850,7 @@ struct ceph_cap_flush *ceph_alloc_cap_flush(void)
 {
 	struct ceph_cap_flush *cf;
 
-	cf = kmem_cache_alloc(ceph_cap_flush_cachep, GFP_KERNEL);
+	cf = kmem_cache_alloc(ceph_cap_flush_cachep, GFP_NOFS);
 	if (!cf)
 		return NULL;