Rename SHRINK_STOP to SPL_SHRINK_STOP

Rename SHRINK_STOP to SPL_SHRINK_STOP to avoid namespace conflicts with
Lustre. On older kernels, SPL defines SHRINK_STOP to -1 for backward
compatbility with the return value of the shrinker implementation
predating the Linux 3.12 shrinker API changes. The conflict arises
because Lustre's 3.12 compatbility code defines SHRINK_STOP to 0 for
older kernels (which is consistent with the Linux value), and their
osd-zfs module (indirectly) includes SPL's linux/mm_compat.h header.
This leads to a 'SHRINK_STOP redefined' build error. Add a SPL_ prefix
to resolve the conflict.

Signed-off-by: Ned Bass <bass6@llnl.gov>