backup: calculate size only once

We need to calculate size so we get an idea of actual used disk space
(which is closer to how much maximum space will be used in the backup,
in case files have huge holes).  Calculate it once to avoid errors.

backup.py

@@ -140,6 +140,7 @@ class Backup:
             st = os.lstat(path)
             is_dir = stat.S_ISDIR(st.st_mode)
             is_reg = stat.S_ISREG(st.st_mode)
+            size = st.st_blocks * 512
 
             # Decorated path ends with a '/' if it's a directory.
             decorated_path = path
@@ -160,14 +161,14 @@ class Backup:
                 # Crosses a mount point
                 exclude_reason = ('I', "skipping, on different filesystem")
 
-            elif (self.config.max_file_size
-                  and is_reg
-                  and (st.st_blocks * 512) > self.config.max_file_size):
+            elif (is_reg
+                  and self.config.max_file_size
+                  and size > self.config.max_file_size):
                 # Too big
                 def format_size(n):
                     return humanfriendly.format_size(
                         n, keep_width=True, binary=True)
-                a = format_size(st.st_blocks * 512)
+                a = format_size(size)
                 b = format_size(self.config.max_file_size)
                 exclude_reason = ('W', f"file size {a} exceeds limit {b}")