xfs: move on-disk inode allocation out of xfs_ialloc()

xfs: move on-disk inode allocation out of xfs_ialloc()

Source kernel commit: 1abcf261016e12246e1f0d2dada9c5c851a9ceb7

So xfs_ialloc() will only address in-core inode allocation then,
Also, rename xfs_ialloc() to xfs_dir_ialloc_init() in order to
keep everything in xfs_inode.c under the same namespace.

[sandeen: make equivalent change in xfsprogs]

Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Gao Xiang <hsiangkao@redhat.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>