bufmgr: Fix ordering of checks in PinBuffer()

bufmgr: Fix ordering of checks in PinBuffer()

The check for skip_if_not_valid added in 819dc118c0f6 was put at the start of
the loop. A CAS loop in theory does allow to make that check in a race free
manner. However, just after the check, there's a
    old_buf_state = WaitBufHdrUnlocked(buf);
which introduces a race, because it would allow BM_VALID to be cleared, after
the skip_if_not_valid check.

Fix by restarting the loop after WaitBufHdrUnlocked().

Reported-by: Yura Sokolov <y.sokolov@postgrespro.ru>
Discussion: https://postgr.es/m/5bf667f3-5270-4b19-a08f-0facbecdff68@postgrespro.ru