xfs: document log sysfs attributes in testing ABI

Create a sysfs-fs-xfs ABI documentation file for newly added sysfs
attributes. This is created under the testing section.

Signed-off-by: Brian Foster <[e-mail]>
Reviewed-by: Dave Chinner <[e-mail]>
Signed-off-by: Dave Chinner <[e-mail]>

Documentation/ABI/testing/sysfs-fs-xfs

···
+What:		/sys/fs/xfs/<disk>/log/log_head_lsn
+Date:		July 2014
+KernelVersion:	3.17
+Contact:	[e-mail]
+Description:
+		The log sequence number (LSN) of the current head of the
+		log. The LSN is exported in "cycle:basic block" format.
+Users:		xfstests
+
+What:		/sys/fs/xfs/<disk>/log/log_tail_lsn
+Date:		July 2014
+KernelVersion:	3.17
+Contact:	[e-mail]
+Description:
+		The log sequence number (LSN) of the current tail of the
+		log. The LSN is exported in "cycle:basic block" format.
+
+What:		/sys/fs/xfs/<disk>/log/reserve_grant_head
+Date:		July 2014
+KernelVersion:	3.17
+Contact:	[e-mail]
+Description:
+		The current state of the log reserve grant head. It
+		represents the total log reservation of all currently
+		outstanding transactions. The grant head is exported in
+		"cycle:bytes" format.
+Users:		xfstests
+
+What:		/sys/fs/xfs/<disk>/log/write_grant_head
+Date:		July 2014
+KernelVersion:	3.17
+Contact:	[e-mail]
+Description:
+		The current state of the log write grant head. It
+		represents the total log reservation of all currently
+		oustanding transactions, including regrants due to
+		rolling transactions. The grant head is exported in
+		"cycle:bytes" format.
+Users:		xfstests