Btrfs: don't WARN() in btrfs_transaction_abort() for IO errors

btrfs_transaction_abort() has a WARN() to help us nail down whatever
problem lead to the abort.  But most of the time, we're aborting for EIO,
and the warning just adds noise.
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/ctree.h

@@ -3445,9 +3445,14 @@ do {								\
 	/* Report first abort since mount */			\
 	if (!test_and_set_bit(BTRFS_FS_STATE_TRANS_ABORTED,	\
 			&((trans)->fs_info->fs_state))) {	\
+		if ((errno) != -EIO) {				\
 			WARN(1, KERN_DEBUG				\
 			"BTRFS: Transaction aborted (error %d)\n",	\
 			(errno));					\
+		} else {						\
+			pr_debug("BTRFS: Transaction aborted (error %d)\n", \
+				  (errno));			\
+		}						\
 	}							\
 	__btrfs_abort_transaction((trans), __func__,		\
 				  __LINE__, (errno));		\