xfs: move inode fork definitions to a new header file

The inode fork definitions are a combination of on-disk format
definition and in-memory tracking and manipulation. They are both
shared with userspace, so move them all into their own file so
sharing is easy to do and track.  This removes all inode fork
related information from xfs_inode.h.

Do the same for the all the C code that currently resides in
xfs_inode.c for the same reason.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>