ima: add cond_resched() in ima_calc_file_hash_tfm loop

When hashing large files, the while loop in ima_calc_file_hash_tfm
processes PAGE_SIZE chunks without any scheduling point, which can
cause soft lockup warnings:
watchdog: BUG: soft lockup - CPU#0 stuck for 50s!
Call Trace:
  _sha256_update+0x12d/0x1a0
  ima_calc_file_hash_tfm+0xfb/0x150
  ima_calc_file_hash+0x6e/0x160
  ima_collect_measurement+0x202/0x340
  process_measurement+0x3a9/0xb30
  ima_file_check+0x56/0xa0
  do_open+0x11b/0x250
  path_openat+0x10b/0x1d0
  do_filp_open+0xa9/0x150
  do_sys_openat2+0x223/0x2a0
  __x64_sys_openat+0x54/0xa0
  do_syscall_64+0x59/0x110
  entry_SYSCALL_64_after_hwframe+0x78/0xe2

Call cond_resched() every 4MB to yield the CPU when needed, rather
than at every loop iteration, to reduce overhead.

Fixes: 3323eec921ef ("integrity: IMA as an integrity service provider")
Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Reviewed-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>