Patch for unix-2.7.2.1 causes boot failure with ghc-8.0.2

[dbaynard]

I've been getting this error preventing ghcjs from booting, corresponding to the following patch. I've been using stack with lts-8.16 (in a clean docker environment).

System/Posix/IO/Common.hsc:284:1: error:
    parse error (possibly incorrect indentation or mismatched brackets)
cabal: Error: some packages failed to install:
directory-1.3.0.0 depends on unix-2.7.2.1 which failed to install.
ghc-boot-8.0.2 depends on unix-2.7.2.1 which failed to install.
ghci-8.0.2 depends on unix-2.7.2.1 which failed to install.
process-1.4.3.0 depends on unix-2.7.2.1 which failed to install.
unix-2.7.2.1 failed during the building phase. The exception was:
ExitFailure 1

diff --git a/System/Posix/IO/Common.hsc b/System/Posix/IO/Common.hsc
index a838352..8e7e317 100644
--- a/System/Posix/IO/Common.hsc
+++ b/System/Posix/IO/Common.hsc
@@ -281,7 +281,7 @@ mode2Int SeekFromEnd  = (#const SEEK_END)
 -- | May throw an exception if this is an invalid descriptor.
 fdSeek :: Fd -> SeekMode -> FileOffset -> IO FileOffset
 fdSeek (Fd fd) mode off =
-  throwErrnoIfMinus1 "fdSeek" (Base.c_lseek fd off (mode2Int mode))
+  throwErrnoIfMinus1 "fdSeek" (fmap fromIntegral $ Base.c_lseek fd (fromIntegral off) (mode2Int mode))
 
 -- -----------------------------------------------------------------------------
 -- Locking

Going to have a play and see whether I can get it to compile, but I don't really get what's going on here.