elide some syscalls in posix backend

[lunixbochs]

Part of #920

posix GetObject before:

newfstatat(dfd: CWD, filename: 0x4692d8, statbuf: 0xc0001c6fa8)       = 0
getxattr(pathname: 0x469300, name: 0x469302, value: 0xc000214c00, size: 1024) = -1 ENODATA
newfstatat(dfd: CWD, filename: 0x469310, statbuf: 0xc0001c7078)       = 0
newfstatat(dfd: CWD, filename: 0x469320, statbuf: 0xc0001c7148)       = 0
listxattr(pathname: 0x469338, list: 0x2036c60)                        = 0
getxattr(pathname: 0x469348, name: 0x469360, value: 0xc000215000, size: 1024) = -1 ENODATA
getxattr(pathname: 0x469378, name: 0x47eab0, value: 0xc000215400, size: 1024) = -1 ENODATA
openat(dfd: CWD, filename: 0x469380, flags: RDONLY|CLOEXEC)           = 8

posix GetObject after:

getxattr(pathname: 0x472378, name: 0x472380, value: 0xc000540000, size: 1024) = -1 ENODATA
newfstatat(dfd: CWD, filename: 0x472398, statbuf: 0xc0001ce9f8)       = 0
listxattr(pathname: 0x4723b0, list: 0x2036c60)                        = 0
openat(dfd: CWD, filename: 0x4723b7, flags: RDONLY|CLOEXEC)           = 8

[versity-github]

This won't automatically run in continuous integration without approval. A member of the Versity organization must allow it.

[benmcclelland]

There is one functional test failure here:

RUN  Versioning_GetObject_invalid_versionId
23:27:49 | 200 |     520.578µs | 127.0.0.1 | PUT | /test-bucket-336 | - | 
23:27:49 | 200 |     562.627µs | 127.0.0.1 | PUT | /test-bucket-336 | - | versioning=
23:27:49 | 200 |     923.169µs | 127.0.0.1 | PUT | /test-bucket-336/my-obj | - | x-id=PutObject
23:27:49 | 404 |      505.13µs | 127.0.0.1 | GET | /test-bucket-336/my-obj | - | versionId=invalid_version_id&x-id=GetObject
FAIL Versioning_GetObject_invalid_versionId: expected error code to be InvalidArgument, instead got NoSuchBucket
23:27:49 | 200 |     653.606µs | 127.0.0.1 | GET | /test-bucket-336 | - | versions=
23:27:49 | 204 |     578.587µs | 127.0.0.1 | DELETE | /test-bucket-336/my-obj | - | versionId=01JB84XAGYRTQH4X6TGV2DZMB0&x-id=DeleteObject
23:27:49 | 204 |     561.755µs | 127.0.0.1 | DELETE | /test-bucket-336 | - | 

I haven't looked to see if we have enough context where there error is being handled here to return the correct errors.