fs/9p: Fix a datatype used with V9FS_DIRECT_IO

The commit in Fixes has introduced some "enum p9_session_flags" values
larger than a char.
Such values are stored in "v9fs_session_info->flags" which is a char only.

Turn it into an int so that the "enum p9_session_flags" values can fit in
it.

Fixes: 6deffc89 ("fs/9p: Add new mount modes")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Dominique Martinet <asmadeus@codewreck.org>
Reviewed-by: Christian Schoenebeck <linux_oss@crudebyte.com>
Signed-off-by: Eric Van Hensbergen <ericvh@kernel.org>