erofs: kill use_vmap module parameter

erofs: kill use_vmap module parameter

As Christoph said [1],
"vm_map_ram is supposed to generally behave better.  So if
it doesn't please report that that to the arch maintainer
and linux-mm so that they can look into the issue.  Having
user make choices of deep down kernel internals is just
a horrible interface.

Please talk to maintainers of other bits of the kernel
if you see issues and / or need enhancements. "

Let's redo the previous conclusion and kill the vmap
approach.

[1] https://lore.kernel.org/r/20190830165533[email protected]/
Reported-by: Christoph Hellwig <[email protected]>
Signed-off-by: Gao Xiang <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>