xfs: report proper f_files in statfs if we overshoot imaxpct

Normally, a statfs syscall reports m_maxicount as f_files
(total file nodes in file system) because it is supposed
to be the upper limit for dynamically-allocated inodes.

It's possible, however, to overshoot imaxpct / m_maxicount.
If this happens, we should report the actual number of allocated
inodes, which is contained in sb_icount.  Add one more adjustment
to the statfs code to make this happen.

Reported-by: Alexander Tsvetkov <alexander.tsvetkov@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>