xfs: fold xfs_mount-alloc() into xfs_init_fs_context()

After switching to use the mount-api the only remaining caller of
xfs_mount_alloc() is xfs_init_fs_context(), so fold xfs_mount_alloc()
into it.

Signed-off-by: Ian Kent <raven@themaw.net>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>