lib/posix-sysinfo: Use posix-fdtab for SC_OPEN_MAX

lib/posix-sysinfo: Use posix-fdtab for SC_OPEN_MAX

This change makes `sysinfo` take the size of the fdtab from Kconfig
options of posix-fdtab instead of a macro defined by vfscore.
This correctly reports the size of the file descriptor table, instead of
the vfscore hardcoded default.

Signed-off-by: Andrei Tatar <andrei@unikraft.io>
Reviewed-by: Radu Nichita <radunichita99@gmail.com>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
GitHub-Closes: #1455