Fix possibly uninitialized 'root_inode' variable warning

Resolve an uninitialized variable warning when compiling.

    In function ‘zfs_domount’:
    warning: ‘root_inode’ may be used uninitialized in this
        function [-Wmaybe-uninitialized]
    sb->s_root = d_make_root(root_inode);

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #11306

module/os/linux/zfs/zfs_vfsops.c

@@ -1451,7 +1451,7 @@ int
 zfs_domount(struct super_block *sb, zfs_mnt_t *zm, int silent)
 {
 	const char *osname = zm->mnt_osname;
-	struct inode *root_inode;
+	struct inode *root_inode = NULL;
 	uint64_t recordsize;
 	int error = 0;
 	zfsvfs_t *zfsvfs = NULL;