block: Fixes nfs compiling error on msys2/mingw

block: Fixes nfs compiling error on msys2/mingw

These compiling errors are fixed:
../block/nfs.c:27:10: fatal error: poll.h: No such file or directory
   27 | #include <poll.h>
      |          ^~~~~~~~
compilation terminated.

../block/nfs.c:63:5: error: unknown type name 'blkcnt_t'
   63 |     blkcnt_t st_blocks;
      |     ^~~~~~~~
../block/nfs.c: In function 'nfs_client_open':
../block/nfs.c:550:27: error: 'struct _stat64' has no member named 'st_blocks'
  550 |     client->st_blocks = st.st_blocks;
      |                           ^
../block/nfs.c: In function 'nfs_get_allocated_file_size':
../block/nfs.c:751:41: error: 'struct _stat64' has no member named 'st_blocks'
  751 |     return (task.ret < 0 ? task.ret : st.st_blocks * 512);
      |                                         ^
../block/nfs.c: In function 'nfs_reopen_prepare':
../block/nfs.c:805:31: error: 'struct _stat64' has no member named 'st_blocks'
  805 |         client->st_blocks = st.st_blocks;
      |                               ^
../block/nfs.c: In function 'nfs_get_allocated_file_size':
../block/nfs.c:752:1: error: control reaches end of non-void function [-Werror=return-type]
  752 | }
      | ^

On msys2/mingw, there is no st_blocks in struct _stat64 yet, we disable the usage of it
on msys2/mingw, and create a typedef long long blkcnt_t; for further implementation

Signed-off-by: Yonggang Luo <luoyonggang@gmail.com>
Message-Id: <20201105123116.674-2-luoyonggang@gmail.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>