Google is your friend: Be nice and visit him often. He can answer questions a lot more quickly than posting them here...
A very quick search using just DMA_BIT_MASK as the search term gave 23,000 hits:
Google DMA_BIT_MASK[
^]
The top hit took me to Linux Crsoss reference,
http://lxr.free-electrons.com/ident?v=2.6.34;i=DMA_BIT_MASK[
^] which links to the definition:
http://lxr.free-electrons.com/source/include/linux/dma-mapping.h?v=2.6.34#L65[
^] which shows the whole macro.
A quick glance tells you exactly what it does! I.e. converts a bit number to a bit mask.
In future, please try to do at least basic research yourself, and not waste your time or ours.