xfs: fully initialize xfs_da_args in xchk_directory_blocks

While running the online fsck test suite, I noticed the following
assertion in the kernel log (edited for brevity):

XFS: Assertion failed: 0, file: fs/xfs/xfs_health.c, line: 571
------------[ cut here ]------------
WARNING: CPU: 3 PID: 11667 at fs/xfs/xfs_message.c:104 assfail+0x46/0x4a [xfs]
CPU: 3 PID: 11667 Comm: xfs_scrub Tainted: G        W         5.19.0-rc7-xfsx #rc7 6e6475eb29fd9dda3181f81b7ca7ff961d277a40
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.15.0-1 04/01/2014
RIP: 0010:assfail+0x46/0x4a [xfs]
Call Trace:
 <TASK>
 xfs_dir2_isblock+0xcc/0xe0
 xchk_directory_blocks+0xc7/0x420
 xchk_directory+0x53/0xb0
 xfs_scrub_metadata+0x2b6/0x6b0
 xfs_scrubv_metadata+0x35e/0x4d0
 xfs_ioc_scrubv_metadata+0x111/0x160
 xfs_file_ioctl+0x4ec/0xef0
 __x64_sys_ioctl+0x82/0xa0
 do_syscall_64+0x2b/0x80
 entry_SYSCALL_64_after_hwframe+0x46/0xb0

This assertion triggers in xfs_dirattr_mark_sick when the caller passes
in a whichfork value that is neither of XFS_{DATA,ATTR}_FORK.  The cause
of this is that xchk_directory_blocks only partially initializes the
xfs_da_args structure that is passed to xfs_dir2_isblock.  If the data
fork is not correct, the XFS_IS_CORRUPT clause will trigger.  My
development branch reports this failure to the health monitoring
subsystem, which accesses the uninitialized args->whichfork field,
leading the the assertion tripping.  We really shouldn't be passing
random stack contents around, so the solution here is to force the
compiler to zero-initialize the struct.

Found by fuzzing u3.bmx[0].blockcount = middlebit on xfs/1554.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>