kill fs/read_write.h

fs/compat.c doesn't need it anymore, so let's just move the remaining
contents (two typedefs) into fs/read_write.c

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --cc fs/compat.c
Simple merge

diff --cc fs/read_write.c
Simple merge

diff --cc fs/read_write.h
Simple merge