staging: vboxsf: fix dereference of pointer dentry before it is null checked

Currently the pointer dentry is being dereferenced before it is
being null checked.  Fix this by only dereferencing dentry once
we know it is not null.

Addresses-Coverity: ("Dereference before null check")
Fixes: df402865 ("staging: Add VirtualBox guest shared folder (vboxsf) support")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20191105175108.79824-1-colin.king@canonical.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/vboxsf/utils.c

@@ -174,7 +174,7 @@ int vboxsf_stat_dentry(struct dentry *dentry, struct shfl_fsobjinfo *info)
 
 int vboxsf_inode_revalidate(struct dentry *dentry)
 {
-	struct vboxsf_sbi *sbi = VBOXSF_SBI(dentry->d_sb);
+	struct vboxsf_sbi *sbi;
 	struct vboxsf_inode *sf_i;
 	struct shfl_fsobjinfo info;
 	struct timespec64 prev_mtime;
@@ -187,6 +187,7 @@ int vboxsf_inode_revalidate(struct dentry *dentry)
 	inode = d_inode(dentry);
 	prev_mtime = inode->i_mtime;
 	sf_i = VBOXSF_I(inode);
+	sbi = VBOXSF_SBI(dentry->d_sb);
 	if (!sf_i->force_restat) {
 		if (time_before(jiffies, dentry->d_time + sbi->o.ttl))
 			return 0;