staging: exfat: Rename variable 'ModifyTimestamp' to 'modify_timestamp'

Fix checkpatch warning: Avoid CamelCase
Change all occurrences of identifier "ModifyTimestamp" to
"modify_timestamp"

Signed-off-by: Pragat Pandya <pragat.pandya@gmail.com>
Link: https://lore.kernel.org/r/20200210183558.11836-19-pragat.pandya@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>