Don't clobber postmaster sigmask in dsm_impl_resize.

Commit 4518c798 intended to block signals in regular backends that
allocate DSM segments, but dsm_impl_resize() is also reached by
dsm_postmaster_startup().  It's not OK to clobber the postmaster's
signal mask, so only manipulate the signal mask when under the
postmaster.

Back-patch to all releases, like 4518c798.

Discussion: https://postgr.es/m/CA%2BhUKGKNpK%3D2OMeea_AZwpLg7Bm4%3DgYWk7eDjZ5F6YbozfOf8w%40mail.gmail.com

src/backend/storage/ipc/dsm_impl.c

@@ -362,6 +362,7 @@ dsm_impl_posix_resize(int fd, off_t size)
 	 * allowed SIGUSR1 to interrupt us repeatedly (for example, due to recovery
 	 * conflicts), the retry loop might never succeed.
 	 */
+	if (IsUnderPostmaster)
 		PG_SETMASK(&BlockSig);
 
 	/* Truncate (or extend) the file to the requested size. */
@@ -402,9 +403,12 @@ dsm_impl_posix_resize(int fd, off_t size)
 	}
 #endif							/* HAVE_POSIX_FALLOCATE && __linux__ */
 
+	if (IsUnderPostmaster)
+	{
 		save_errno = errno;
 		PG_SETMASK(&UnBlockSig);
 		errno = save_errno;
+	}
 
 	return rc;
 }