ceph: try getting buffer capability for readahead/fadvise

For readahead/fadvise cases, caller of ceph_readpages does not
hold buffer capability. Pages can be added to page cache while
there is no buffer capability. This can cause data integrity
issue.

Signed-off-by: Yan, Zheng <zyan@redhat.com>

fs/ceph/caps.c

@@ -2479,6 +2479,27 @@ static void check_max_size(struct inode *inode, loff_t endoff)
 		ceph_check_caps(ci, CHECK_CAPS_AUTHONLY, NULL);
 }
 
+int ceph_try_get_caps(struct ceph_inode_info *ci, int need, int want, int *got)
+{
+	int ret, err = 0;
+
+	BUG_ON(need & ~CEPH_CAP_FILE_RD);
+	BUG_ON(want & ~(CEPH_CAP_FILE_CACHE|CEPH_CAP_FILE_LAZYIO));
+	ret = ceph_pool_perm_check(ci, need);
+	if (ret < 0)
+		return ret;
+
+	ret = try_get_cap_refs(ci, need, want, 0, true, got, &err);
+	if (ret) {
+		if (err == -EAGAIN) {
+			ret = 0;
+		} else if (err < 0) {
+			ret = err;
+		}
+	}
+	return ret;
+}
+
 /*
  * Wait for caps, and take cap references.  If we can't get a WR cap
  * due to a small max_size, make sure we check_max_size (and possibly