lib-index: Put back some of the removed locking complexity. This fixes a crash when mmap_disable=no and a index was mmap()ed (which keeps the index locked) and later its read-lock was tried to be changed to write-lock.
lib-index: Put back some of the removed locking complexity.
This fixes a crash when mmap_disable=no and a index was mmap()ed (which
keeps the index locked) and later its read-lock was tried to be changed to
write-lock.

damoxc/dovecot,damoxc/dovecot,damoxc/dovecot,damoxc/dovecot,damoxc/dovecot