client: Add permissions checking to open

client: Add permissions checking to open

Users of the libcephfs api (fuse in particular)
don't check the mode against the open flags.  This
commit does the proper checks to grant/deny access
to the file.  The check_mode() function constructs
a requested mode based on the flags, and compares that
to the mode of the file.

Signed-off-by: Sam Lang <sam.lang@inktank.com>