vboxsf: Honor excl flag to the dir-inode create op

vboxsf: Honor excl flag to the dir-inode create op

Honor the excl flag to the dir-inode create op, instead of behaving
as if it is always set.

Note the old behavior still worked most of the time since a non-exclusive
open only calls the create op, if there is a race and the file is created
between the dentry lookup and the calling of the create call.

While at it change the type of the is_dir parameter to the
vboxsf_dir_create() helper from an int to a bool, to be consistent with
the use of bool for the excl parameter.

Fixes: 37a2c12c9093 ("fs: Add VirtualBox guest shared folder (vboxsf) support")
Signed-off-by: Hans de Goede <hdegoede@redhat.com>