file: Qualify pointers to capsicum rights as const

File descriptor lookup routines typically take a set of capsicum rights
as input to the lookup, so that the fd's rights can be atomically
checked. This set should be qualified with const.

No functional change intended.

Reviewed by: olce, oshogbo, brooks, kib
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D50419