fs/ext*,f2fs,jffs2,reiserfs: give comments for aclsize and count calculation

I agree that defining a constant for this makes sence. Just please be
more verbose with the comment. This is hardly readable for anyone not knowing what's going on in advance. Also braces around 4 are superfluous. Add something like the comment below before posix_acl_valid():

/*
* posix_acl_valid() makes sure ACL format is the following:
* ACL_USER_OBJ ACL_USER*[1] ACL_GROUP_OBJ ACL_GROUP*[1] ACL_MASK[2] ACL_OTHER *
* [1] Where * is the regexp sense of "0 or more times"
* [2] If ACL_USER or ACL_GROUP is present, then ACL_MASK must be present. */

Then add comment before definition of ACL_MAX_SHORT_ENTRY:
/*
* posix_acl_valid() makes sure that if there are <= 4 ACL entries, then
* all of them are short. Otherwise exactly 4 entries are short ones and
* other have full length. See comment before that function for exact ACL
* format.
*/