lib/ukvmem: Allow non-writable shared file mappings

lib/ukvmem: Allow non-writable shared file mappings

Non-writable shared file mappings are treated as anonymous read-only file
mappings. Note that any writes made to the underlying file will not be
reflected in memory.

Signed-off-by: Cosmin Vancea <csvancea@gmail.com>
Reviewed-by: Andra Paraschiv <andra@unikraft.io>
Approved-by: Marc Rittinghaus <marc.rittinghaus@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #832