swiotlb: Expose swiotlb_nr_tlb function to modules

As a mechanism to detect whether SWIOTLB is enabled or not.
We also fix the spelling - it was swioltb instead of
swiotlb.

CC: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
[v1: Ripped out swiotlb_enabled]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/swiotlb-xen.c

@@ -153,7 +153,7 @@ void __init xen_swiotlb_init(int verbose)
 	char *m = NULL;
 	unsigned int repeat = 3;
 
-	nr_tbl = swioltb_nr_tbl();
+	nr_tbl = swiotlb_nr_tbl();
 	if (nr_tbl)
 		xen_io_tlb_nslabs = nr_tbl;
 	else {

include/linux/swiotlb.h

@@ -24,7 +24,7 @@ extern int swiotlb_force;
 
 extern void swiotlb_init(int verbose);
 extern void swiotlb_init_with_tbl(char *tlb, unsigned long nslabs, int verbose);
-extern unsigned long swioltb_nr_tbl(void);
+extern unsigned long swiotlb_nr_tbl(void);
 
 /*
  * Enumeration for sync targets

lib/swiotlb.c

@@ -110,11 +110,11 @@ setup_io_tlb_npages(char *str)
 __setup("swiotlb=", setup_io_tlb_npages);
 /* make io_tlb_overflow tunable too? */
 
-unsigned long swioltb_nr_tbl(void)
+unsigned long swiotlb_nr_tbl(void)
 {
 	return io_tlb_nslabs;
 }
-
+EXPORT_SYMBOL_GPL(swiotlb_nr_tbl);
 /* Note that this doesn't work with highmem page */
 static dma_addr_t swiotlb_virt_to_bus(struct device *hwdev,
 				      volatile void *address)
@@ -321,6 +321,7 @@ void __init swiotlb_free(void)
 		free_bootmem_late(__pa(io_tlb_start),
 				  PAGE_ALIGN(io_tlb_nslabs << IO_TLB_SHIFT));
 	}
+	io_tlb_nslabs = 0;
 }
 
 static int is_swiotlb_buffer(phys_addr_t paddr)