f2fs: super: constify fscrypt_operations structure

Declare fscrypt_operations structure as const as it is only stored in
the s_cop field of a super_block structure. This field is of type const,
so fscrypt_operations structure having this property can be made const
too.

File size before: fs/f2fs/super.o
   text	   data	    bss	    dec	    hex	filename
  54131	  31355	    184	  85670	  14ea6	fs/f2fs/super.o

File size after: fs/f2fs/super.o
   text	   data	    bss	    dec	    hex	filename
  54227	  31259	    184	  85670	  14ea6	fs/f2fs/super.o

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>