xfs: don't clear the "dinode core" in xfs_inode_alloc

The xfs_icdinode structure just contains a random mix of inode field,
which are all read from the on-disk inode and mostly not looked at
before reading the inode or initializing a new inode cluster.  The
only exceptions are the forkoff and blocks field, which are used
in sanity checks for freshly allocated inodes.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>