dma-buf: Cleanup comments on dma_buf_map_attachment()

Mappings need to be unmapped by calling dma_buf_unmap_attachment() and
not by calling again dma_buf_map_attachment(). Also fix some spelling
mistakes.

Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171101140630.2884-1-Liviu.Dudau@arm.com

drivers/dma-buf/dma-buf.c

@@ -351,13 +351,13 @@ static inline int is_dma_buf_file(struct file *file)
  *
  * 2. Userspace passes this file-descriptors to all drivers it wants this buffer
  *    to share with: First the filedescriptor is converted to a &dma_buf using
- *    dma_buf_get(). The the buffer is attached to the device using
+ *    dma_buf_get(). Then the buffer is attached to the device using
  *    dma_buf_attach().
  *
  *    Up to this stage the exporter is still free to migrate or reallocate the
  *    backing storage.
  *
- * 3. Once the buffer is attached to all devices userspace can inniate DMA
+ * 3. Once the buffer is attached to all devices userspace can initiate DMA
  *    access to the shared buffer. In the kernel this is done by calling
  *    dma_buf_map_attachment() and dma_buf_unmap_attachment().
  *
@@ -617,7 +617,7 @@ EXPORT_SYMBOL_GPL(dma_buf_detach);
  * Returns sg_table containing the scatterlist to be returned; returns ERR_PTR
  * on error. May return -EINTR if it is interrupted by a signal.
  *
- * A mapping must be unmapped again using dma_buf_map_attachment(). Note that
+ * A mapping must be unmapped by using dma_buf_unmap_attachment(). Note that
  * the underlying backing storage is pinned for as long as a mapping exists,
  * therefore users/importers should not hold onto a mapping for undue amounts of
  * time.