vfs: fall back to fstat() if stat() is not implemented

[benpicco]

Contribution description

Instead of making the application work around this (as #14397 did) implement this in vfs so that vfs_stat() always works independent of the filesystem.

Testing procedure

TODO: Which filesystem implements fstat() but not stat()?

Issues/PRs references

[chrysn]

TODO: Which filesystem implements `fstat()` but not `stat()`?

If it's not certain that any fs needs this at all, might the better way be requiring fstat to be present when stat is present, and if needed, provide a generic stat impl that FSs can place in their driver?

[benpicco]

Oh we already have vfs_sysop_stat_from_fstat (used by fatfs).
And spiffs (which is the only one that lacks a .stat implementation) provides SPIFFS_stat(), it's just not hooked up.