mount.ceph: remove arbitrary limit on size of name= option

mount.ceph: remove arbitrary limit on size of name= option

Aaron was getting back -ERANGE errors when trying to mount using
a long name= option. The issue is that the destination buffer for the
"key=" string is not big enough to hold long names.

When I overhauled the mount.ceph code recently, I made this buffer much
smaller than before figuring that it didn't need to be any larger than
the length of "secret=<base64 encoded key>".

In the case where the secret is set in the keyring though, this buffer
needs to be able to hold a string like "key=client.<cephx name>". The
cephx name can be of arbitrary length, however.

Rework the code to just safe_cat the strings directly into the options
buffer, eliminating an extra copy and the need for an arbitrary limit.
This also allows us to remove get_secret_option() from the the common
code as well.

Fixes: https://tracker.ceph.com/issues/43649
Reported-by: Aaron <aarongmldt@gmail.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
(cherry picked from commit 8db2a44c9438749be98d41fb309f10d5084805df)

Conflicts:
	src/common/secret.c