xlog: fix fallocate vs read race

posix_fallocate(), which is used for preallocating disk space for WAL
files, increases the file size and fills the allocated space with zeros.
The problem is a WAL file may be read by a relay thread at the same time
it is written to. We try to handle the zeroed space in xlog_cursor (see
xlog_cursor_next_tx()), however this turns out to be not enough, because
transactions are written not atomically so it may occur that a writer
writes half a transaction when a reader reads it. Without fallocate, the
reader would stop at EOF until the rest of the transaction is written,
but with fallocate it reads zeroes instead and thinks that the xlog file
is corrupted while actually it is not.

Fix this issue by using fallocate() with FALLOC_FL_KEEP_SIZE flag
instead of posix_fallocate(). With the flag fallocate() won't increase
the file size, it will only allocate disk space beyond EOF.

Closes #3883