reiserfs: Remove VLA from fs/reiserfs/reiserfs.h

Remove Variable Length Array from fs/reiserfs/reiserfs.h. EMPTY_DIR_SIZE
is used as an array size and as it is using strlen() it need not be
evaluated at compile time. Change it's definition to use sizeof() to
force evaluation of array length at compile time.

Signed-off-by: Kyle Spiers <kyle@spiers.me>
Signed-off-by: Jan Kara <jack@suse.cz>