qa/cephfs: fix invalid escape sequence SyntaxWarnings

qa/cephfs: fix invalid escape sequence SyntaxWarnings

python 3.12 warns on unrecognized escape sequences in string literals.
fuse_mount's admin-socket pyscript and mount's nft payload carry regex
and shell escapes (\., \d, \;) that are meant literally, so make those
strings raw. filesystem's get_mds_addr docstring had a stray \/ in the
example address, which is just a slash, so drop the backslash.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>