s3: VFS: Change SMB_VFS_READLINK to use const struct smb_filename * instead of const...

s3: VFS: Change SMB_VFS_READLINK to use const struct smb_filename * instead of const char *.

We need to migrate all pathname based VFS calls to use a struct
to finish modernising the VFS with extra timestamp and flags parameters.

Signed-off-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Richard Sharpe <realrichardsharpe@gmail.com>