ext4: remove unused variable warning in parse_options()

ext4: remove unused variable warning in parse_options()

Commit c33fbe8f673c5 ("ext4: Enable blocksize < pagesize for
dioread_nolock") removed the only user of 'sbi' outside of the ifdef,
so it caused a new warning:

fs/ext4/super.c:2068:23: warning: unused variable 'sbi' [-Wunused-variable]

Fixes: c33fbe8f673c5 ("ext4: Enable blocksize < pagesize for dioread_nolock")
Signed-off-by: Olof Johansson <olof@lixom.net>
Link: https://lore.kernel.org/r/20191111022523.34256-1-olof@lixom.net
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Ritesh Harjani <riteshh@linux.ibm.com>