Introduce "struct stat_ex" as a replacement for SMB_STRUCT_STAT

Introduce "struct stat_ex" as a replacement for SMB_STRUCT_STAT

This patch introduces

struct stat_ex {
        dev_t           st_ex_dev;
        ino_t           st_ex_ino;
        mode_t          st_ex_mode;
        nlink_t         st_ex_nlink;
        uid_t           st_ex_uid;
        gid_t           st_ex_gid;
        dev_t           st_ex_rdev;
        off_t           st_ex_size;
        struct timespec st_ex_atime;
        struct timespec st_ex_mtime;
        struct timespec st_ex_ctime;
        struct timespec st_ex_btime; /* birthtime */
        blksize_t       st_ex_blksize;
        blkcnt_t        st_ex_blocks;
};
typedef struct stat_ex SMB_STRUCT_STAT;

It is really large because due to the friendly libc headers playing macro
tricks with fields like st_ino, so I renamed them to st_ex_xxx.

Why this change? To support birthtime, we already have quite a few #ifdef's at
places where it does not really belong. With a stat struct that we control, we
can consolidate the nanosecond timestamps and the birthtime deep in the VFS
stat calls.

At this moment it is triggered by a request to support the birthtime field for
GPFS. GPFS does not extend the system level struct stat, but instead has a
separate call that gets us the additional information beyond posix. Without
being able to do that within the VFS stat calls, that support would have to be
scattered around the main smbd code.

It will very likely break all the onefs modules, but I think the changes will
be reasonably easy to do.