[GCC4] fix build error in arch/alpha/kernel/osf_sys.c

[GCC4] fix build error in arch/alpha/kernel/osf_sys.c

This patches fixes this error with gcc 4 on alpha :

gcc-4.1 -D__KERNEL__ -I/data/git/public/linux-2.4/include -Wall -Wstrict-prototypes -Wno-trigraphs -O2 -fno-strict-aliasing -fno-common -fno-builtin-sprintf -fomit-frame-pointer -Wno-pointer-sign -pipe -mno-fp-regs -ffixed-8 -mcpu=ev6 -Wa,-mev6   -nostdinc -iwithprefix include -DKBUILD_BASENAME=osf_sys  -c -o osf_sys.o osf_sys.c
osf_sys.c: In function 'osf_filldir':
osf_sys.c:128: error: invalid lvalue in assignment