ANDROID: Incremental fs: fix minor printk format warning

Use the correct printk specifier [%zu] for size_t variable.
This fixes the following warning:

fs/incfs/format.c: In function ‘incfs_read_next_metadata_record’:
./include/linux/kern_levels.h:5:18: warning: format ‘%ld’ expects argument of type ‘long int’, but argument 2 has type ‘size_t’ {aka ‘unsigned int’} [-Wformat=]
fs/incfs/format.c:669:3: note: in expansion of macro ‘pr_warn’
  669 |   pr_warn("incfs: The record is too large. Size: %ld",
      |   ^~~~~~~

Bug: 183339614
Fixes: c6819dd77861 (ANDROID: Initial commit of Incremental FS)
Reported-by: kernelci.org bot <bot@kernelci.org>
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Change-Id: Ia784a9ced9fb6bc76e2f1baa495b3ccf568e3b1d