LU-13127 ptlrpc: avoid ISO C90 mixed declaration

LU-13127 ptlrpc: avoid ISO C90 mixed declaration

Avoid forbidden mixed declaration in lustre_msg_calc_cksum()

Test-Parameters: trivial
Fixes: 726897c87c42 ("LU-13127 ptlrpc: don't require CONFIG_CRYPTO_CRC32")
Signed-off-by: Mikhail Pershin <mpershin@whamcloud.com>
Change-Id: I180eefd979331f3fa206aa9b1d54c8e7b17bcbee
Reviewed-on: https://review.whamcloud.com/39677
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Neil Brown <neilb@suse.de>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>