Fix abd_enter/exit_critical wrappers

Commit fc551d7 introduced the wrappers abd_enter_critical() and
abd_exit_critical() to mark critical sections.  On Linux these are
implemented with the local_irq_save() and local_irq_restore() macros
which set the 'flags' argument when saving.  By wrapping them with
a function the local variable is no longer set by the macro and is
no longer properly restored.

Convert abd_enter_critical() and abd_exit_critical() to macros to
resolve this issue and ensure the flags are properly restored.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #10332

module/zfs/abd.c

@@ -703,7 +703,7 @@ abd_raidz_gen_iterate(abd_t **cabds, abd_t *dabd,
 	struct abd_iter caiters[3];
 	struct abd_iter daiter = {0};
 	void *caddrs[3];
-	unsigned long flags = 0;
+	unsigned long flags __maybe_unused = 0;
 
 	ASSERT3U(parity, <=, 3);
 
@@ -800,7 +800,7 @@ abd_raidz_rec_iterate(abd_t **cabds, abd_t **tabds,
 	struct abd_iter citers[3];
 	struct abd_iter xiters[3];
 	void *caddrs[3], *xaddrs[3];
-	unsigned long flags = 0;
+	unsigned long flags __maybe_unused = 0;
 
 	ASSERT3U(parity, <=, 3);