This function can be frequently called with MAY_EXEC|MAY_NOT_BLOCK
during RCU path walk. Where possible we should try not to break
out of it. In this case we check whether flag ZFS_NO_EXECS_DENIED is
set and check mode (similar to fastexecute check in zfs_acl.c).
Signed-off-by: Andrew Walker <awalker@ixsystems.com>