Maintain valid md.c state when FileClose() fails.
FileClose() failure ordinarily causes a PANIC. Suppose the user
disables that PANIC via data_sync_retry=on. After mdclose() issued a
FileClose() that failed, calls into md.c raised SIGSEGV. This fix adds
repalloc() calls during mdclose(); update a comment about ignoring
repalloc() cost. The rate of relation segment count change is a minor
factor; more relevant to overall performance is the rate of mdclose()
and subsequent re-opening of segments. Back-patch to v10, where commit
45e191e3aa62d47a8bc1a33f784286b2051f45cb introduced the bug.
Reviewed by Kyotaro Horiguchi.
Discussion: https://postgr.es/m/
20191222091930.GA1280238@rfd.leadboat.com