xfs: avoid unused to_mp() function warning

to_mp() was first introduced with the following commit:
'commit 801cc4e1 ("xfs: debug mode forced buffered write failure")'

But the user of to_mp() was removed by below commit:
'commit f8c47250 ("xfs: convert drop_writes to use the errortag
mechanism")'

So kernel build with clang throws below warning message:

   fs/xfs/xfs_sysfs.c:72:1: warning: unused function 'to_mp' [-Wunused-function]
   to_mp(struct kobject *kobject)

Hence to_mp() might be removed safely to get rid of warning message.

Signed-off-by: Austin Kim <austindh.kim@gmail.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>