Linux: always check or verify return of igrab()

zhold() wraps igrab() on Linux, and igrab() may fail when the inode 
is in the process of being deleted.  This means zhold() must only be
called when a reference exists and therefore it cannot be deleted. 
This is the case for all existing consumers so add a VERIFY and a
comment explaining this requirement.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Adam Moss <c@yotes.com>
Closes #11704

module/os/linux/zfs/zfs_ctldir.c

@@ -590,7 +590,8 @@ struct inode *
 zfsctl_root(znode_t *zp)
 {
 	ASSERT(zfs_has_ctldir(zp));
-	igrab(ZTOZSB(zp)->z_ctldir);
+	/* Must have an existing ref, so igrab() cannot return NULL */
+	VERIFY3P(igrab(ZTOZSB(zp)->z_ctldir), !=, NULL);
 	return (ZTOZSB(zp)->z_ctldir);
 }
 

module/os/linux/zfs/zfs_vfsops.c

@@ -1734,7 +1734,11 @@ zfs_vget(struct super_block *sb, struct inode **ipp, fid_t *fidp)
 			VERIFY(zfsctl_root_lookup(*ipp, "snapshot", ipp,
 			    0, kcred, NULL, NULL) == 0);
 		} else {
-			igrab(*ipp);
+			/*
+			 * Must have an existing ref, so igrab()
+			 * cannot return NULL
+			 */
+			VERIFY3P(igrab(*ipp), !=, NULL);
 		}
 		ZFS_EXIT(zfsvfs);
 		return (0);

module/os/linux/zfs/zpl_inode.c

@@ -593,7 +593,8 @@ zpl_link(struct dentry *old_dentry, struct inode *dir, struct dentry *dentry)
 
 	crhold(cr);
 	ip->i_ctime = current_time(ip);
-	igrab(ip); /* Use ihold() if available */
+	/* Must have an existing ref, so igrab() cannot return NULL */
+	VERIFY3P(igrab(ip), !=, NULL);
 
 	cookie = spl_fstrans_mark();
 	error = -zfs_link(ITOZ(dir), ITOZ(ip), dname(dentry), cr, 0);