ext4: reserve revoke credits in __ext4_new_inode

It's possible that __ext4_new_inode will release the xattr block, so
it will trigger a warning since there is revoke credits will be 0 if
the handle == NULL. The below scripts can reproduce it easily.

------------[ cut here ]------------
WARNING: CPU: 0 PID: 3861 at fs/jbd2/revoke.c:374 jbd2_journal_revoke+0x30e/0x540 fs/jbd2/revoke.c:374
...
__ext4_forget+0x1d7/0x800 fs/ext4/ext4_jbd2.c:248
ext4_free_blocks+0x213/0x1d60 fs/ext4/mballoc.c:4743
ext4_xattr_release_block+0x55b/0x780 fs/ext4/xattr.c:1254
ext4_xattr_block_set+0x1c2c/0x2c40 fs/ext4/xattr.c:2112
ext4_xattr_set_handle+0xa7e/0x1090 fs/ext4/xattr.c:2384
__ext4_set_acl+0x54d/0x6c0 fs/ext4/acl.c:214
ext4_init_acl+0x218/0x2e0 fs/ext4/acl.c:293
__ext4_new_inode+0x352a/0x42b0 fs/ext4/ialloc.c:1151
ext4_mkdir+0x2e9/0xbd0 fs/ext4/namei.c:2774
vfs_mkdir+0x386/0x5f0 fs/namei.c:3811
do_mkdirat+0x11c/0x210 fs/namei.c:3834
do_syscall_64+0xa1/0x530 arch/x86/entry/common.c:294
...
-------------------------------------

scripts:
mkfs.ext4 /dev/vdb
mount /dev/vdb /mnt
cd /mnt && mkdir dir && for i in {1..8}; do setfacl -dm "u:user_"$i":rx" dir; done
mkdir dir/dir1 && mv dir/dir1 ./
sh repro.sh && add some user

[root@localhost ~]# cat repro.sh
while [ 1 -eq 1 ]; do
    rm -rf dir
    rm -rf dir1/dir1
    mkdir dir
    for i in {1..8}; do  setfacl -dm "u:test"$i":rx" dir; done
    setfacl -m "u:user_9:rx" dir &
    mkdir dir1/dir1 &
done

Before exec repro.sh, dir1 has inherit the default acl from dir, and
xattr block of dir1 dir is not the same, so the h_refcount of these
two dir's xattr block will be 1. Then repro.sh can trigger the warning
with the situation show as below. The last h_refcount can be clear
with mkdir, and __ext4_new_inode has not reserved revoke credits, so
the warning will happened, fix it by reserve revoke credits in
__ext4_new_inode.

Thread 1                        Thread 2
mkdir dir
set default acl(will create
a xattr block blk1 and the
refcount of ext4_xattr_header
will be 1)
				...
                                mkdir dir1/dir1
				->....->ext4_init_acl
				->__ext4_set_acl(set default acl,
			          will reuse blk1, and h_refcount
				  will be 2)

setfacl->ext4_set_acl->...
->ext4_xattr_block_set(will create
new block blk2 to store xattr)

				->__ext4_set_acl(set access acl, since
				  h_refcount of blk1 is 2, will create
				  blk3 to store xattr)

  ->ext4_xattr_release_block(dec
  h_refcount of blk1 to 1)
				  ->ext4_xattr_release_block(dec
				    h_refcount and since it is 0,
				    will release the block and trigger
				    the warning)

Link: https://lore.kernel.org/r/20191213014900.47228-1-yangerkun@huawei.com


Reported-by: Hulk Robot <hulkci@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: yangerkun <yangerkun@huawei.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>