Ensure required subdirectories of /var/fm/fmd exist

[citrus-it]

When a blank ZFS dataset is created to back /var/fm/fmd, the fault management daemon creates the files it needs but it does not create subdirectories that are required to store data. sled-agent needs to ensure these exist after mounting the dataset

Fixes #4444

[citrus-it]

I tested this on a bench gimlet with no pre-existing pools.

gimlet-sn06 # tail -F `svcs -L sled-agent` | looker -c 'r.component == "BackingFs"'
16:06:49.602Z INFO SledAgent (BackingFs): Processing fmd
    file = sled-agent/src/backing_fs.rs:126
16:06:49.602Z INFO SledAgent (BackingFs): Ensuring dataset oxi_a462a7f7-b628-40fe-80ff-4e4189e2d62b/backing/fmd
    file = sled-agent/src/backing_fs.rs:136
16:06:49.871Z INFO SledAgent (BackingFs): Stopping service svc:/system/fmd:default
    file = sled-agent/src/backing_fs.rs:170
16:06:51.042Z INFO SledAgent (BackingFs): Mounting oxi_a462a7f7-b628-40fe-80ff-4e4189e2d62b/backing/fmd on /var/fm/fmd
    file = sled-agent/src/backing_fs.rs:178
16:06:51.092Z INFO SledAgent (BackingFs): Ensuring directory /var/fm/fmd/rsrc
    file = sled-agent/src/backing_fs.rs:186
16:06:51.093Z INFO SledAgent (BackingFs): Ensuring directory /var/fm/fmd/ckpt
    file = sled-agent/src/backing_fs.rs:186
16:06:51.093Z INFO SledAgent (BackingFs): Ensuring directory /var/fm/fmd/xprt
    file = sled-agent/src/backing_fs.rs:186
16:06:51.093Z INFO SledAgent (BackingFs): Starting service svc:/system/fmd:default
    file = sled-agent/src/backing_fs.rs:192

gimlet-sn06 # ls -CF /var/fm/fmd
ckpt/           fltlog          infolog_hival   xprt/
errlog          infolog         rsrc/

[jgallagher]

This is completely fine as-is, but I'll just note that you could do Option<&'static [&'static str]> here to avoid the <'a> bound on BackingFs. (Doing so would be less flexible than what you have, but since everything else in here has a 'static bound we could make this match.)