sh: remove CONFIG_SET_FS support

sh uses set_fs/get_fs only in one file, to handle address
errors in both user and kernel memory.

It already has an abstraction to differentiate between I/O
and memory, so adding a third class for kernel memory fits
into the same scheme and lets us kill off CONFIG_SET_FS.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>