Documentation: filesystem: Convert xfs.txt to ReST

Move xfs.txt to admin-guide, convert xfs.txt to ReST and broken references
Signed-off-by: Sheriff Esseson <sheriffesseson@gmail.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

Documentation/admin-guide/index.rst

@@ -70,6 +70,7 @@ configure specific aspects of kernel behavior to your liking.
    ras
    bcache
    ext4
+   xfs
    pm/index
    thunderbolt
    LSM/index

Documentation/filesystems/dax.txt

@@ -76,7 +76,7 @@ exposure of uninitialized data through mmap.
 These filesystems may be used for inspiration:
 - ext2: see Documentation/filesystems/ext2.txt
 - ext4: see Documentation/filesystems/ext4/
-- xfs:  see Documentation/filesystems/xfs.txt
+- xfs:  see Documentation/admin-guide/xfs.rst
 
 
 Handling Media Errors

MAINTAINERS

@@ -17356,9 +17356,8 @@ L:	linux-xfs@vger.kernel.org
 W:	http://xfs.org/
 T:	git git://git.kernel.org/pub/scm/fs/xfs/xfs-linux.git
 S:	Supported
-F:	Documentation/filesystems/xfs.txt
+F:	Documentation/admin-guide/xfs.rst
 F:	Documentation/ABI/testing/sysfs-fs-xfs
-F:	Documentation/filesystems/xfs.txt
 F:	Documentation/filesystems/xfs-delayed-logging-design.txt
 F:	Documentation/filesystems/xfs-self-describing-metadata.txt
 F:	fs/xfs/