ext4: avoid lockdep warning when inheriting encryption context

ext4: avoid lockdep warning when inheriting encryption context

On a lockdep-enabled kernel, xfstests generic/027 fails due to a lockdep
warning when run on ext4 mounted with -o test_dummy_encryption:

    xfs_io/4594 is trying to acquire lock:
     (jbd2_handle
    ){++++.+}, at:
    [<ffffffff813096ef>] jbd2_log_wait_commit+0x5/0x11b

    but task is already holding lock:
     (jbd2_handle
    ){++++.+}, at:
    [<ffffffff813000de>] start_this_handle+0x354/0x3d8

The abbreviated call stack is:

 [<ffffffff813096ef>] ? jbd2_log_wait_commit+0x5/0x11b
 [<ffffffff8130972a>] jbd2_log_wait_commit+0x40/0x11b
 [<ffffffff813096ef>] ? jbd2_log_wait_commit+0x5/0x11b
 [<ffffffff8130987b>] ? __jbd2_journal_force_commit+0x76/0xa6
 [<ffffffff81309896>] __jbd2_journal_force_commit+0x91/0xa6
 [<ffffffff813098b9>] jbd2_journal_force_commit_nested+0xe/0x18
 [<ffffffff812a6049>] ext4_should_retry_alloc+0x72/0x79
 [<ffffffff812f0c1f>] ext4_xattr_set+0xef/0x11f
 [<ffffffff812cc35b>] ext4_set_context+0x3a/0x16b
 [<ffffffff81258123>] fscrypt_inherit_context+0xe3/0x103
 [<ffffffff812ab611>] __ext4_new_inode+0x12dc/0x153a
 [<ffffffff812bd371>] ext4_create+0xb7/0x161

When a file is created in an encrypted directory, ext4_set_context() is
called to set an encryption context on the new file.  This calls
ext4_xattr_set(), which contains a retry loop where the journal is
forced to commit if an ENOSPC error is encountered.

If the task actually were to wait for the journal to commit in this
case, then it would deadlock because a handle remains open from
__ext4_new_inode(), so the running transaction can't be committed yet.
Fortunately, __jbd2_journal_force_commit() avoids the deadlock by not
allowing the running transaction to be committed while the current task
has it open.  However, the above lockdep warning is still triggered.

This was a false positive which was introduced by: 1eaa566d368b: jbd2:
track more dependencies on transaction commit

Fix the problem by passing the handle through the 'fs_data' argument to
ext4_set_context(), then using ext4_xattr_set_handle() instead of
ext4_xattr_set().  And in the case where no journal handle is specified
and ext4_set_context() has to open one, add an ENOSPC retry loop since
in that case it is the outermost transaction.

Signed-off-by: Eric Biggers <ebiggers@google.com>