staging: erofs: remove __EROFS_BIT

It's better to use pre-calculated values to make
on-disk definition more straight-forward and human-readable.

Since there is the only one user, let's remove
__EROFS_BIT entirely.

Suggested-by: Joe Perches <joe@perches.com>
Signed-off-by: Gao Xiang <hsiangkao@aol.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>