posix: Add fs_file_t type variable initializations

The commit adds initializations of fs_file_t variables in preparation
for fs_open function change that will require fs_file_t object, passed
to the function, to be initialized before first usage.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>

lib/posix/fs.c

@@ -107,7 +107,7 @@ int open(const char *name, int flags, ...)
 		return -1;
 	}
 
-	(void)memset(&ptr->file, 0, sizeof(ptr->file));
+	fs_file_t_init(&ptr->file);
 
 	rc = fs_open(&ptr->file, name, zmode);