fs: allocate large buffer to avoid attr size check

[alexdotc]

This halves the number of xattr calls by allocating a buffer of 64 bytes, which should be (more than) sufficient for all attrs we're checking across all directories on ceph.

I also switched to a fized-size stack array, but honestly it probably doesn't matter that much; it works with a vec if we move the buf.set_len() call further down the unsafe block so that we can use the return value of lgetxattr.

[lgarrison]

Looks great, thanks!