dma-buf: replace dma_buf_export() with dma_buf_export_named()

For debugging purposes, it is useful to have a name-string added
while exporting buffers. Hence, dma_buf_export() is replaced with
dma_buf_export_named(), which additionally takes 'exp_name' as a
parameter.

For backward compatibility, and for lazy exporters who don't wish to
name themselves, a #define dma_buf_export() is also made available,
which adds a __FILE__ instead of 'exp_name'.

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
  [Thanks for the idea!]
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>

Documentation/dma-buf-sharing.txt

@@ -52,14 +52,23 @@ The dma_buf buffer sharing API usage contains the following steps:
    associated with this buffer.
 
    Interface:
-      struct dma_buf *dma_buf_export(void *priv, struct dma_buf_ops *ops,
-				     size_t size, int flags)
+      struct dma_buf *dma_buf_export_named(void *priv, struct dma_buf_ops *ops,
+				     size_t size, int flags,
+				     const char *exp_name)
 
    If this succeeds, dma_buf_export allocates a dma_buf structure, and returns a
    pointer to the same. It also associates an anonymous file with this buffer,
    so it can be exported. On failure to allocate the dma_buf object, it returns
    NULL.
 
+   'exp_name' is the name of exporter - to facilitate information while
+   debugging.
+
+   Exporting modules which do not wish to provide any specific name may use the
+   helper define 'dma_buf_export()', with the same arguments as above, but
+   without the last argument; a __FILE__ pre-processor directive will be
+   inserted in place of 'exp_name' instead.
+
 2. Userspace gets a handle to pass around to potential buffer-users
 
    Userspace entity requests for a file-descriptor (fd) which is a handle to the