xfs: zero entire directory data block header region at init

xfs: zero entire directory data block header region at init

xfs_dir3_data_init currently zeroes only the xfs_dir3_blk_hdr portion of
the directory data block header, then manually initializes the bestfree
entries in a loop. This leaves the pad field in xfs_dir3_data_hdr
uninitialized and requires explicit zeroing of each bestfree slot.

Zero the entire header region (geo->data_entry_offset bytes)
unconditionally before setting individual fields. This covers all
current and future header fields, all padding (implicit and explicit),
and the bestfree array, so the manual zeroing loop for bestfree can be
removed.

Suggested-by: Dave Chinner <dgc@kernel.org>
Signed-off-by: Yuto Ohnuki <ytohnuki@amazon.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>