xfs: drop IDONTCACHE on inodes when we mark them sick

When we decide to mark an inode sick, clear the DONTCACHE flag so that
the incore inode will be kept around until memory pressure forces it out
of memory.  This increases the chances that the sick status will be
caught by someone compiling a health report later on.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>