xfs: switch to use the new mount-api

Define the struct fs_parameter_spec table that's used by the new
mount-api for options parsing.

Create the various fs context operations methods and define the
fs_context_operations struct.

Create the fs context initialization method and update the struct
file_system_type to utilize it. The initialization function is
responsible for working storage initialization, allocation and
initialization of file system private information storage and for
setting the operations in the fs context.

Also set struct file_system_type .parameters to the newly defined
struct fs_parameter_spec options parsing table for use by the fs
context methods and remove unused code.

[darrick: add a comment pointing out the one place where mp->m_super is
null]

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>