Switch KM_SLEEP to KM_PUSHPAGE

Under certain circumstances the following functions may be called
in a context where KM_SLEEP is unsafe and can result in a deadlocked
system.  To avoid this problem the unconditional KM_SLEEPs are
converted to KM_PUSHPAGEs.  This will prevent them from attempting
to initiate any I/O during direct reclaim.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

include/sys/taskq.h

@@ -62,6 +62,7 @@ typedef struct taskq_ent {
  */
 #define TQ_SLEEP                KM_SLEEP
 #define TQ_NOSLEEP              KM_NOSLEEP
+#define TQ_PUSHPAGE             KM_PUSHPAGE
 #define TQ_NOQUEUE              0x01000000
 #define TQ_NOALLOC              0x02000000
 #define TQ_NEW                  0x04000000