new primitive: __fs_parse()

fs_parse() analogue taking p_log instead of fs_context.
fs_parse() turned into a wrapper, callers in ceph_common and rbd
switched to __fs_parse().

As the result, fs_parse() never gets NULL fs_context and neither
do fs_context-based logging primitives

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>